After years of inactivity I decided to release two old project, which I started to work on in 2010, publicly on my GitHub account. I have not produced any prototypes yet, so they are currently more a draft. Both projects use a flash EPROM to store the data in the cartridge. The first one is a simple Amstrad Plus / Amstrad GX4000 cartridge, which can be equipped with an original ACID protection chip or with a Xilinx XC9536 CPLD for ACID emulation (you can read more about the emulation here).
The second project is the design of a flash cartridge, which can be programmed on a PC via USB and used in a Amstrad Plus / Amstrad GX4000. You can read more about the design in the README.md.
Maybe someone with more time is interested to take over the projects, otherwise I will work on them every now and then…