I'm currently working on a atari multicart (similar in function to ultimate cart) in order to pick up some new skills (vhdl, pcb routing, ...) and it's probably finally in a state when i can send a few pieces into the world for testing and feedback
It supports .car files up to 1MB (the most common bankswitching types should be supported, i'll be adding new ones as i get hands on some test files for them) and .xex files.
It's housed in a "commodore" shell so it doesn't fit 1200XL (but i've been told it can be used when removed from the shell) and cover on 400/800 can't be closed (shell too tall).
Cart can selfupdate from the sd card so unless something goes terribly wrong there's no need for special hw to update its firmware.
Here's a short video showing the basic functionality
I have currently 3 extra carts built, if anyone is interested, they're available for 25EUR each (+5EUR p&p).
Feel free to ask anything and any feedback is welcome.