What seat pan have you sourced?
Looking at what you have there says that it was a liftoff seat. The mechanisms for the liftoff seat have been removed though you still have the slots in the forward section of the frame for the fork under the pan to insert into. Keep the fork handy as it may become an important piece of a seat mod.
Back to the question....... What seat pan? The '78/'79 hinged Special seats are the most aesthetically pleasing, IMHO, and can be adapted to your situ.....thinking I've got a couple of 'em collecting dust around here. No foam or covers, just pans.
What grab rail are you using? Might affect the direction you go in the mod.