It has taken longer than expected, but I am happy to announce I have a final version of an RCA Studio II Multicart. The newer system board versions were crippling a signal on the cartridge port, which meant that larger games (using $c00 and higher) would not work. Thanks to Ed Keefe (also active here), the cart now fixes this signal, meaning that no modification is needed to the console itself!
Using 2 hex encoders, 128 slots are available on an EEPROM - this is plenty for the existing games, both official ones and homebrew. The cartridge is compatible with clones, such as the Victory MPT-02 (though not with the Toshiba Visicom, as the slot/pinout is different). Should new games come out, or found, they can be added by flashing the EEPROM. Some photos are attached showing the cart playing Pong and Asteroids (both games by Paul Robson).
It's been designed to fit the original cartridge shells. Two holes are needed in the front for the rotary selection switches (and some internal tabs need to go as well). I have some spare shells of broken carts, courtesy of toymailman. I probably won't have time to modify them for the carts, but I'll add the shells on a first-come basis (i.e. until they run out).
The design is open source - see the schematics and the board file attached. You're welcome to build your own of course, but these are not intended to be used in a commercial product. If you'd rather have a ready-made one, I am happy to ship one anywhere in the world for US$40. Leave your username, whether you need a shell (on first come basis only - you'll have to modify it yourself to fit the cart). For those interested, I can add a small PCB that gives you composite out at no extra cost (but you need some soldering skills to add it to the console of course). Put that in your post as well...
I'll keep a list below this post with orders, to keep track of things. When your cart is ready to ship, I'll send you a PM with further details.
Any questions, feel free to ask!