Re: homemade seats
I built my own seats, complete interior actually, on 2 boats and have received comments at the dock. It wasn't 100% showroom quality, but it looked pretty darn good.
I don't think anyone will have "plans" as most of us who build their own seats are trying to make them fit their boat. On my old SeaSwirl I designed the interior with a big open space in the middle, no back-back seats so there was plenty of room for walking around. Plus, LOTS of storage. If you need stock seats, it's probably cheaper and easier to buy them as suggested earlier.
If you are going to build your own, only use thick materials where needed (seat tops) or you will add a lot of unnecessary weight to the boat. Then you will need to seal it all up so water does not ruin it in short order. Then selecting the right density of foam for the seats and for the seat backs, they need to be different for comfort. Then buying "marine" grade vinyl, stitching it up, stapling it down, pulling it tight so it looks right... It can be done, but it's not going to be easy,,, if it's going to look good and last
One of the worst mistakes I see often is people over-building things for the boat and weighting it down. Using 3/4" ply where 1/4", 3/8" or 1/2" would have worked fine... Or, throwing in a bunch of 2x4's as if they were building a house.