CAN'T USE CARTCONV.EXE TO MAKE A .BIN FILE THAT WORKS. Ooops.
Viewing post in SUB-X C64 Updated to V1.2. comments
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.