Hoods are the final extra that turns a catsuit into a full-coverage suit, or 'Zentai' as it's often known. (If you're curious about Zentai, take a look at Ayus Zentai for a gentle introduction - they're very friendly and safely on the other side of the glass.)
From what I can see of the photographs on the net, most suits' hoods follow a very simple pattern and seek to minimise seams, particularly around the face. I present a very simple hood for if your fabric has 50% stretch, and a more complex one that will fit better and will also work with less stretchy fabrics.
I'd suggest NOT using the same scaling factor you're using for the rest of the suit (in much the same way as I suggest not scaling the feet or hands), unless you particularly enjoy your nose being squished (which I don't - hoods aren't really my thing).
This hood (pictured above) makes heavy use of the stretchiness of the fabric (at least 50%), so you'll need to experiment a little to find out what's comfortable, and it won't work with all fabrics. My head's about 5.5" in radius, and I used a radius of 5", which looked fine but because I didn't do the EF cut/spread, my nose was a little uncomfortable.
- Measure the circumference of your head, just near your ears as if you're wearing glasses. Divide by 6 to get the approximate radius.
- Draw a circle of that radius, then draw two diameters at right-angles (AD, BC in the diagram).
- Where they intersect the circle (A and B), mark your fold-line, and extend it by the length of your neck (to G).
- Measure out from G to half the circumference of your neck. (Note this will NOT necessarily end up precisely under D, it just happens to for my diagram.)
- Cut across EF and separate the parts by about an inch (you'll need to experiment with this, and you may want to make the CFD curve a little more generous for your nose's sake - move F out half an inch, say).
- Cut out.
- Cut up from G to B, straight up to A (ignore E), around the top to C, around the curve to F and then D, and vertically down from D along that blue line.
- If you want a zip, put it in along CFD and down the back of the neck, sewing normally along AC.
- For a pull-on hood, sew ACFD, but be careful to check that the neck can stretch enough for your head, with at least an inch to spare.
It's a pity that the neck attaches into the back of the head, because that puts the complex shapes at the front where you don't want a seam - shaping is easiest when you can make it with a seam.
This is a hood pattern that has a better natural fit than the trivial pattern, so it'll work with less stretchy fabrics. In its four-panel form (abcd) it has a seam centrally down the forehead, but a hood with only three panels (fgh) will be a slightly worse fit. If you're feeling clever, you could shuffle abc along half a panel, and split d so each half is on either side (the pale blue dotted lines at the top of the diagram).
This pattern derives from the silly fleece hat.
A run-down of the measurements used in the diagram to the side:
|Sc||Scalp circumference - measure around where your glasses rest.|
|Sh||Scalp height - measure from ear to ear over the top of your head and divide by two. This is almost certainly your scalp circumference divided by four.|
|Fh||Face height - from your ears (scalp circumference) to the corner of your jaw.|
|Cw||Chin width - actually, probably your closed mouth's width|
|Jl||Jaw length - from your chin to the corner of your jaw.|
|Nc||Neck circumference - measure around your neck.|
|Nh||Neck height - measure from your jaw to your collar.|
The petals of the panels are based on a smooth line joining five points as given in the table below. Just like the leg measurements are plotted on either side of the centre line for the catsuit pattern, so the heights here are just to be plotted along the centre line of each panel, and the widths should be plotted with half on either side of the centre line at those heights. The angle of the corner at the top of each panel is 90 degrees for 4-panel, 120 for 3-panel, etc.
- Mark (cut a notch) to locate the centre of the chin piece with the centre of the head piece.
- Sew the jaw into the bottom of panels b and c (or g), then sew the two vertical throat seams.
- Sew the common seam of petal a and b together, and the seam of c and d (or f and g, then g and h).
- One long seam from the forehead (or top of head) to the nape of the neck remains. Install a zip into this with its open end at the nape of the neck. The zip should be between Nh+Fh and Nh+Fh+Sh in length.
(For those who care about such things, I worked out the horizontal width of a great circle at heights with conveniently simple cosines, hence the circumference of the perpendicular circles at those heights, which should be shared between the panels you want to use.)
Note - the table gives the width each panel should have. Divide the width in two and plot it either side of each panel's centre line. Although this calculator assumes equal division of the values, your panels needn't all be the same fraction, so long as their widths at height 0 all add up to a full circumference - for instance, you could have three 1/5 panels and one 2/5 panel at the front. (You might notice that the two-panel pattern based on these calculations results in a total circumference of Sc, whereas the simple two-circle hood above only has a circumference of 4*Sr, about 2/3*Sc - that's why the simple hood needs a fabric with 50% stretch.)
Changing the neckline
To make a zentai suit, you'll want the hood to replace the collar. (Unless you're making an origami zentai suit.)
It's probably easier to make the hood as a separate piece and sew it to the neckline.
You should use a body pattern with a seam running up the back - the conventional style would be best - and you'll probably want to install a zip in the back of the hood (see the zips page) but most zentai suits seem to have two zips sharing that back seam - one up from below, and one down from above. Since the collar can't stretch, the overall zipped length will need to be a bit longer than it would otherwise be.
You might consider trying to use shoulder-zips with this, instead of a central back zip.
If you're using in-set sleeves, it's easy to merge a variant of the circular hood (with seams running around the sides of the face and over the ears) into the neckline, and merge the hood's seam with the shoulder-top seam, but you won't have a good place for a zip.
It's surprising how much one can see through stretched lycra, but it's messy to drink through it, and you might want clearer vision anyway.
I haven't tried this, but...
To make an opening in the hood you can probably just cut the lycra - I've never found full-stretch lycra to run - but make sure you cut the holes small, because they're likely to stretch rather more than you might expect, particularly over time. You can try to counteract that by sewing a tight narrow zig-zag around the hole.
You could alternatively try placing a second piece of lycra where you want the hole, right sides together, sewing around the edge, and THEN cutting the hole out and pushing the extra piece through before sewing it in place and trimming the excess away.