To get you started, here is the circuit board to make your own CV cartridge. Just throw a 28 pin socket, a 100nF 0805 capacitor and a 74LS21D on the board, set the two 32K solder jumpers and you're ready to plug in a 27C256 EPROM containing a game of your choice (of up to 32K, which includes all games originally made for the ColecoVision). You can have circuit boards made by just sending the above Gerber files to a company making circuit boards (e.g. JLCPCB).