Jeff, this is a future nessmuking article that I just haven't had time to finish. It's still a draft and will be finished in the future. It's rough and some of the thoughts not completely fleshed out. This might help out:

A well-designed kayak cockpit makes the difference between a full day of comfort and the feeling of wanting to sell the kayak at a bargain basement price. Whether building a new boat or buying a commercially available design, the following criteria should be considered.

Cockpit Height

The side of a cockpit coaming should come no higher than the top of the kayaker’s hip bone. This allows for freedom of movement in torso rotation. It also often keeps the rear cockpit coaming low enough to allow for easy rear deck rolls. A lower cockpit rim also makes it easier to sit into the seat and pull your legs in after you. This is a nice feature to utilize during launch and a nice feature to reverse when landing on rocky beaches.
During building, the height can be adjusted by including a recess of some type for the coaming. Some designs will have coaming rims higher than the top of your hip bones, and this can be easily avoided by choosing another design.

Cockpit Length and Width

The length and width of a cockpit will partly be determined by your choice of ocean or keyhole. Ocean cockpits are often cited as better because they require a smaller skirt and lessen the chance of implosion from waves. Ocean cockpits may also help people with poor eyesight see their maps easier, because an ocean cockpit is shorter and the map can be kept closer. In addition, it could be argued that ocean cockpit provide greater contact with the boat and thus control more control over the kayak. Generally, most ocean cockpit are 16” by 20”.
For most paddlers though, a keyhole cockpit is ideal. It’s easier getting in and out of. Good thigh braces provide excellent contact to control the boat, and modern neoprene skirts will stand up to plenty of surf without imploding. The only disadvantage is that the map must be stored further away.
With a keyhole cockpit, make sure that the front of the cockpit is long enough so that the legs can be pulled in easily after sitting down. This helps during launching and landing and also helps on the water when reentering. The width is also important in this aspect. It needs to be wide enough to be able to drop into quickly. Keyhole cockpits very in design and size, but a good size is 16 inches wide by 31 inches long.

Constant Curve

The coaming lip needs to be shaped so that it is in a constant curve with no flat areas. When the spray skirt is in contact with a constant curve it will be tensioned its whole circumference, which will keep it tight up against the coaming riser. This will result in fewer leaks.

Knee Height

The height of the front of the cockpit and in the knee area is also important. A coaming and front deck that is too high will result in the banging of knuckles during paddle stokes. It can also compress the bending of the knees. Most people find that by sitting in a diamond shape with their legs in the kayak that they will be more comfortable. A very low deck forces the knees and legs into a straight out position which can stress the hamstrings. This can be very uncomfortable. A front coaming height of around 12” generally is low enough to avoid banging the knuckles, high enough to be comfortable, and it makes getting into and out of the kayak easy.

Knee and Thigh Braces

It’s important to have solid contact with the kayak. Thigh or knee braces provide contact for the top portion of the knees, and this helps with kayak edging, rolling, and generally stability. Some boats provide an additional brace or protrusion to fit knees and others just narrow a bit at the thighs.
A good thigh brace shouldn’t just connect with the paddler at the knees, but that contact...