After successfully bending purchased flats, I'm considering making one from scratch.

I believe the purchased flat was 4130 steel.  Heat treating was necessary and easy.

What other alloys would also be acceptable?

I do happen to have some stainless steel.  It is about 2mm thick (about twice as thick as a regular receiver).  I don't know exactly what type of stainless it is, but it came from a piece of counter from a commercial kitchen.  It will certainly be difficult to work with and maybe impossible to bend as needed, but if that can be overcome, will this be suitable for a reciever?  My plan was to keep the inside dimensions the same and maybe fabricate longer pins.  If this works, a coarse blasted surface may look good on it.  

Purchasing another flat is probably the best thing to do, but just in case I feel like doing things the hard way, I want to do it right.

Also, solid steel rivets aren't exactly something I can find anywhere locally.  I know I can order some from many sources, but I only need a few.  I know AK rivet kits can be purchased cheap, but just incase I decide to do things the hard way...   I can easily make rivets on the lathe.  Would a big steel nail be soft enough?  

Another complication is I'd like to undo a screw build and do it with rivets.  AK rivet kits probably won't work well for this.  Something a little thicker might be needed.

Hey, skip the stainless, It could be done, but the flat bending jigs probably aren't up to bending that thickness.  4130 is avalable.  Look up "metal express" on google.  You can get enough to build 4 guns for about $20 or so.  Rivets are avalable if you look long enough.  Google again, or get 'em from someone here or at gunco. Turn the special ones on your lathe.  Not worth looking for.  Nails make good rivet stock, have used them before.  Good luck.