When I first got my Atari 800 in the 80's, I used it to develop some games + Utility software.
I realised there was a 4K hole in the system that could be used to put debugging software etc.
I rarely played Computer Chess, so that cartridge got the chop.
Also nothing used the right side cartridge slot.
I found a 4K RAM chip with the the same pinout as the ROM, did a little rewire of the cartridge and the 800 motherboard cartridge slot
to enable writing to the RAM.
It worked a treat, obviously couldn't use it with Basic.
At a later date I did make a 'Cartridge' for the Chess ROM.
See attached photo's