I got a weird problem involving both of my cats (Siegfrieda, to the left; Kika, to the right).
Kika is rather particular about having her own litterbox(es), and refuses to use a litterbox shared by another cat. Frieda on the other hand is adept to the "if I fits, I sits, I shits" philosophy, and is totally OK sharing litterboxes.
That creates a problem: no matter if properly and regularly cleaned, the only one using litterboxes here is Frieda. We had, like, five of them at once; and Kika would still rather do her business on the patio.
How do I either teach Kika "it's fine to share a litterbox", or teach Siegfrieda "that's Kika's litterbox, leave it alone"?
This was the first thing that came up in a search. Looks like there's a few sizes too:
https://meowspace.biz/product/meowspace-microchip-system/
It looks pretty pricey, but considering microchip pet doors on their own cost about that much, this seems like a cheaper option than DIYing some contraption involving a microchip pet door.
Look around for this kind of stuff OP! It exists!
Op might need some electrical engineering but I'm fairly certain there's a homemade solution using an rfid tag on a collar.