Decades ago, when building 1 of my Pro-Drag bikes (NHRA Div. #6), I had to build a seat. I started with a Factory seat that I stripped to the metal base. I cut & re-welded the base to the shape of my desire. I then took a large block of foam & cut & shaped it to what was required. I purchased a large piece of vinyl & after several "Dry" runs of installing it on my custom built seat, I riveted & GLUED it over the seat. The rivets obviously held the vinyl in place, but the glue not only ALSO helped hold the vinyl, but sealed it as well.
That particular bike was a big bore TSCC Suzuki. I did not cover the entire seat with the glue, rather, mostly the outer perimeter. Also, a few spots where the contours might be apt to pull (the vinyl) or make it let go.
I had a rusted out seat pan on my 1980 GS1000G (sold about 7 years ago with over 100K and still running strong!). Couldn't find a good used one. Looked hard at it, realized the hinges and latch were riveted on, and made one from layered plywood. It was slightly taller on the frame than the stamped steel, about 3/4" (thickness of plywood.. thinner wouldn't be sturdy enough) but it worked great! Put the replacement cover I'd bought a year or two earlier (how I knew it was rusted so bad -- the cover never did hold good on the edges) on it and it looked great! Until you flipped the seat up you couldn't tell. Had a solid base then another layer with the center cut out for the fender and such under the seat. Used original foam. Sat great too! Just had to be creative. I looked at adapting another big bike seat, think I had one from a Honda, but they either were too rusted or didn't fit right without lots of modification.... and they would rust again... Sealed the wood with a couple coats of polyurethane. Still on there and running with the guy I sold it to, and I did this about 2000-2001.