Dumb question….

#10
I usually use 1/2" on seats and if its cantileavered off the back I weld a brace to support it. If it has shocks and has to hinge i make a 1/8" steel plate that holds the hinge and shocks then bolt the seat to the plate. I dont usually use T nuts i make a little plate with nuts welded on that i screw down on the top side of the wood and run the bolts up through to that. Use layered carped padding and spray glue between layers for the foam covered in a thin mattress pad to smooth it out. I make my own pleating just use thin stranded electical wire and a zipper foot with vinyl of choice. How its attatched and supported is more important than the type and thickness. You dont want a bike that has nothing but the seat to pick it up by or the plywood will break eventually. So always make a hoop or something to grab back there.
 
#11
Another suggestion might to go with 3/4" HDO plywood. This is the stuff used to build concrete forms. It's made with solid veneers and manufactured under extremely high pressure. You probably wouldn't want to afford a whole sheet of this for your purpose. That said, it may be possible to get a small scrap piece from a local concrete form contractor. BTW, this stuff also comes in 1 1/8" thickness and is commonly used for those premanufactured form systems. Needless to say, a small piece of that would be bombproof. Also, a plus for using that thicker material might negate the need for additional bracing, particularly in the case of a seat designed with some amount of "frame overhang".
 
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