Once you finally decide what you're going to do.. just make a cardboard template that suits your likes and have it cut from a sheet of 20 gauge..
Then bend it to contour your bike..
Have it upholstered and you're done..
These old jap bikes use seats that have almost flat seat pans.. .
Most other applications have some drop to them..
Trying to salvage used seat pans can be a task..
That seat in my pic is a seat that SBF made for this specific application..
Once you have the fender secured from the sides.. they with withstand a lot of weight.
I've used a bunch of glass fenders with 2 up seats and carried a ton of weight on them.

Build it the way you want it ..... then learn how to ride it like you own it..
www.lowridersbysummers.com