I've mentioned this stuff on Discord, but to spread it around further:
I've put together a KiCad library for expansion cards and cartridges. It includes two different symbols, and then two different footprints. One for cards, and another for carts that has been tested to fit in aftermarket Famicom cartridge cases.
There are also a few other minor symbols available.
That's available at https://github.com/jkominek/x16expansion
I've also just put up a small (one page) "tutorial"/explainer on designing a cartridge.
https://github.com/jkominek/x16simplecart
The schematic and resulting PCB have been tested and work.
If you don't want to go through with it and make your own cartridge, there are also Gerber files you can download and have made for your personal use.