When I was thinking about a bench seat for my Squirt I built a rectangular base out of oak framing and planned to tie it in to the chines as in the plan diagram. It calls for the a 14 inch deep seat. If you follow the chine line and use standard 1" stock it will work fine. For the seat back you should plan to either hinge it for access or block across the aft end of the bottom and add tabs to inset at the bottom so you can lift the bak out. I ended up buying bucket seats; but I would most likely rethink that idea as the 3.5 inch thickness was a little more than I should have used. I would go with 2 inch or less thickness on the seat cushions or just use a 1/4 or 3/8 inch plywood panel. I have posted a pic of my original idea. I hope this helps.
March 30 View of cockpit with seat fitted.jpg [ 2.6 MiB | Viewed 519 times ]