Mine is on an early Featherbed frame. There are two brackets on the rear subframe and a strap that goes across between the rear suspension
top mounts. I have made a rectangular seat mounting frame out of 1" x 1/8" mild steel strap that mounts on the motorcycle frame via four small bolts and is held down by thumb wheels from below as per the original early Dominator seat. When I have made the seat base I will mount it on this sub frame by sticking them together with fibreglass.
As your bike is a Triumph with a hinged seat my suggestion would be to make a metal frame that used the seat hinge points and the seat catch so that it will hinge upwards as per the original. The seat needs to be liftable to get to the battery
and the oil tank. One of the hinge points will need to be removable so that the seat can be fitted and removed. Then fibreglass the seat base to this frame, using spacers as necessary .
Four rubber buffers between the seat (or its frame) and the motorcycle frame will carry the weight of the rider.