You have registered a new boat in your profile I see
Jon, dead easy, took a couple of weekends in all. Start with a sheet of 5.5mm B & Q ply. Doesn't need to be anything else as you are going to cover inside and out with GRP. Try 10m of 450gr CSM and 10 L of resin. Add your chosen colour to the final coat or just use white resin if thats more suitable. The ply is joined with a 2" x 1" frame and the three man seat has a bulkhead half down along. The bottom is about 14" wide and the top 11.5" I think because thats the width of the foam I bought. Total length is 48" which happens to be the width of a sheet of ply. With three layers on the outside and two on the inside I can just lift it into the boat. The back wall is reinforced with thick ply to take the grab rail, which came from a previous rib.
The foam came from Flew Sails in Portchester but most sail makers also make seats and covers so should be able to supply. It comes in a semi circle already which saves trying to cut the stuff as its really diffcult to get a straight edge. Sail makers use something that looks like an industrial carving knife.
Hope this helps