: have not been able to get comfortable with my seat, my legs get numb after
: about half hour of paddling.

: I built the guellemot but need advice on the best seat design for comfort

Hi Michael,
make your seat at least 50 cm (18") long. That solves most problems.
I made mine with two layers of a softer waterproof foam, with a triangular (cross section) strip around the periphery between the two layers and a tringular section (also triangular in cross section) between the legs also between the two layers. The whole lot took 30 minutes to make and put together.
JohnK