I wanted to make some cartridges.
I see that 64 CUPCAKECART uses .prg files,
so if you download the Prg files from here:
https://thereaperuk.itch.io/reapers-mega-cartridge-image-c64
The "TheC64Maxi-Mini Prg Files.zip" download option, you will find Sub-X Prg file in the Zip, So try to use that with the Cupcake board.
My PRG Files are Crunched (Make smaller file size) so still might not work.