The A8PicoCart is a multi-cart for the Atari 8-bit (XL/XE) which you can make at home with only basic soldering skills. It is based on my earlier UnoCart design but uses a very inexpensive Raspberry Pi Pico clone (£2-3) for an easy build.
When plugged into a PC by USB it becomes a Mass Storage device allowing ROM/CAR/XEX & ATR files to be copied to the cartridge. When plugged into an Atari these files are shown on the menu and the device will either emulate the selected cartridge type or act as a XEX file launcher. It also has (very) limited support for ATR files allowing you to do some programming and save your work to an ATR file.
This page contains everything you need to build the project - firmware to flash to your purple Pico clone, Kicad PCB design files to make a PCB and 3d print files for a nice case. The source code for everything is also here. There is also a manual detailing the full capabilities of the cartridge.