Recently I restored a Roland XP-60 synth for a friend. It's a digital sample based synth with possibility of expanding sample set with up to 4 expansion cards. And, while looking at service manual I got carried away a little... Those expansion cards have a pretty straightforward interface - 8 bit data bus, 20 bit address bus, a few chip enable pins. Looked like something very DIYable.
Needless to say, I'm not the first one to notice it. Someone made copies of available expansions and reverse engineered the format. SRJV is a script collection to create/manipulate those ROMs.
For my own clone I used AM29F016D-70E4C flash chips. These are obsolete but still widely available on eBay.
I included extra headers to expose flash WE, RESET for in-circuit programming, but that part is untested - I ended up pre-programming chips before soldering.
Did not want to spend too much time on layout so went with a more expensive 4 layer board.
Two expansions installed next to one original from Roland:
