Procedure

Sit in the boat and measure where your body passes through the plane of the cockpit opening.

The "Conning Tower" Make a conical mandrel as shown for the upper skirt "tower."   I used a piece of thin aluminum sheet joined with duct tape.  The upper circumference should be approximately 4" less than your waist size, the bottom length about 2" longer.  My tower is 11" tall, but I paddle C-1 and sit about 6" higher than in a kayak.  It also depends on how high you want the skirt to go on your torso.  Make legs for the mandrel, or support it on something, so that it will duplicate your body position as it sits in the boat. The neoprene tower (below-right) is pie-shaped, with the inside of the pie trimmed off.  The inside and outside radii can be figured from your neoprene tower height and the lower and upper diameter.  See below. Note that W is your waist size minus 2". You can skip figuring (and measuring) the angle by just drawing a correct radius arc and measuring along it with a tape.  Generally, the total radius (T) will be in the range of 6 feet, so may be able to cheat a little.

Wrap the neoprene around your tower(nylon side in) so that it it fits snugly but isn't stretched.  Trim to desired size and shape.  Don't forget that the tower will overlap the cockpit cover by about 2" so the neoprene must extend past the cockpit plane with the mandrel in the boat.  Use neoprene cement to butt-join the ends of the tower section.. After the joint is dry, add a strip of 1/16" neoprene on the smooth (non-nylon) side to reinforce the joint.  Take the mandrel and neoprene out of the boat if it is still in it.

Make a loop with shock cord.  Make it short enough to keep quite a bit of tension on the skirt.  Leave a loop opposite the joined end and close with a couple of rings at the loop and at the back joint.

Cockpit Cover Lay the neoprene sheet over the cockpit, nylon side down, leaving roughly 4" extending past the cockpit rim..  You will be glueing neoprene-side to neoprene-side. Stretch the shock cord loop over the neoprene, allowing it to pull the neoprene into the cockpit rim.  If it does not do this easily, you may have to shorten the loop where it is butt-joined. Evenly pull the neoprene cover material out so that there is a little tension in the top surface.  Don't try to make it "drum tight", just sag-less.  Too much tension will make the skirt come off at very inopportune times... When the top tension is uniform, trim the outside edge of the neoprene so that when it is turned back against the rim it will overlap (past the shock cord) by about 2".  See the bottom profile diagram.

Because I rub my paddle shaft on the cockpit rim of my paddling side (remember I'm a C-1er), I leave a little more overlap material along the right side of the cockpit for wear protection.   Cut a small slot at the front for the front loop. Using lacquer thinner or acetone, clean the neoprene surface of any talc, etc. You can sand the neoprene lightly for an even better bond. Put 2 coats of neoprene glue on the overlapping edges, all the way around.  Gently stretch the outside edge back over the top surface.  Don't stretch side-to-side - in fact compress it in that direction.  otherwise you'll end up with lots of bunching from the extra material.  If you do get bunching, you can always trim the excess off with scissors, though.

Once the neoprene cement is well set, cut a hole where the mandrel sat before (where your body is going to be), but about 2" smaller than the mandrel diameter.  Stretch the hole and sit the mandrel in the boat with the cockpit neoprene stretched around the mandrel with the seam toward the stern. Slide the completed neoprene tower down over the mandrel (nylon side out) so that it overlaps the bottom by about two inches.  Then roll the bottom edge up so that it's no longer touching.

The finished profile of parts Put 2 coats of neoprene glue on the tower and bottom section.  When tacked off, carefully roll the top down over the bottom. You're done! (The next one will be much neater!)