I built the seats for this Challenger.
http://www.***boat.com/image_center/...hallenger6.jpg
http://www.***boat.com/image_center/...hallenger1.jpg
http://www.***boat.com/image_center/...hallenger5.jpg
Sorry about the pics, I did'nt take pictures during the process.
I started with 1/4 ply and attempted to steam them into a barrel shape for the front buckets, that did not work too well as the wood did not want to hold shape.
I ended up using 1/2 inch marine ply and making the backs of the buckets out of five straight pieces. The trick was test fitting the pieces and getting them set at the proper angles to feel and look right.
The bench seat in the back and the seat bases were straight forward using the same 1/2 marine plywood. I used screws and brackets with through bolted ny-lock nuts to hold the wood together. The foam and vinyl cover the bolt heads and nuts nicely. The side panels were 1/4 ply.
I used 2 inch aluminum angle for brackets to hold the seat bases to the stringers.
I coated all the wood liberally with resin to seal it. Just remember to round the corners on the wood so the corners don't try to tear the vinyl once the upholstery is stitched together. I learned t his one the hard way.
Good luck.