Generally the sit-on-top kayaks built from solid foam seem to be simply carved out of blocks of the material.

You can find 2 inch and thicker foam insulation panels at home centers and builder's supply. Construction cement that comes in a caulking tube which is meant to adhere these panels to walls will not disolve them. You can use a few judicously placed dabs of that to bond several thin panels together to make a thick block to start from.

You'll do less carving if you start with slabs of foam that roughly follow the lines of the boat you are building. For example, if you wnat a boat that is 24 inches wide, you may want to asemble a piece thta is 4 iches thick, but 24 inches wide, and as long as you wish your boat to be. To the top anf bottom of this, center and glue on slabs that are 22 inches wide. for the next layer use slapbs that are 20 inches wide,, then go to pieces that are 16, 10, and 4. If you are working with 2 inch thick material you'll have a "block" that is 14 inches high and 24 inches wide.

The table of offsets from almost any kayak plans will give you a guide as to how wide your foam panels should be at any point along the length. Much as I like the walrus design, the specs on it are for rather widely spaced forms. Another design might give you more info you can use. Offsets from Nick's free stitch and glue Guillemot could be one source.They are on this board. Or there are several books that list offsets of the boats they talk about. I believe "Kayakcraft" might be of interest.

Roughing out the shape on thin ( 2 inch thick is thin? well, in this case it is) panels can be done rather easily with a hot wire foam carving tool (available from craft stores) Cut your material a bit bigger than the specs call for as you'll then have some margin for error when you get to shaping it.

Some coarse sandpaper on a belt sander, and watch the styrene chips fly like snow as you sand down the edges and round out things.

You can save a lot of time by doing your laminations of the foam panels in such a manner that you don't have to carve out the cockpit.

When your done with smoothing the edges, paint on a coat of epoxy resin to seal things, and then put on your glass and resin. Once the foam is sealed with the epoxy resin, if you want you could use polyester resin for covering the glass. I think Spidey has been the person who has mentioned surfboard builders by him doing this type of construction.

Hope this helps.

PGJ