itch.io

Sorry about that, but as i know nothing about making/flashing cartridge's for the C64 i cannot help you as i don't have a clue what to do next, I only made .crt files for TheC64Maxi/Mini and Kung Fu Flash as that is easy, just make magic cart files with CartConv and works every time.

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.