Maybe this will help. I made my own mounts (and seat but that's another story). The seat rail is from 1" square tube. It's 7" long, just because that looked about right, no technical reason. I sawed most of one side of the tube off, to make a long "C" section so I could get at the seat bolts. If I'd had C section I would have used it, but it's a bobber so I just use what's hanging around. A couple holes drilled to the same centres as the seat bolts, slot with a round file for fore/ aft adjustment and it's done.
The hinge bracket is from 1" x 1/8" flat bar. Just a short piece the same dimension as the original seat mounting bracket, holes drilled same place so it fits on the stock tank mount. Bend up a "U" shaped bracket that the seat rail fits inside, drill a hole through the lot of them for a pin. The only welding I needed was attaching the "U" shape to the flat bar. If you didn't want to weld, you could use a nut and bolt or rivet it I guess.
Painted the lot and called it good. Took maybe an hour to do the job and cost $0.00
The back of the seat just rests on the back fender mounting horns, until I get around to sawing them off and fitting hairpin saddle springs. I slipped a short piece of radiator hose over each horn to soak up a few bumps/vibes.
Hope that helps.