Here is a 3D printable case kindly designed by Chrissy. It will house your Atari ST Mega Cartridge (https://backofficeshow.com/) perfectly.
There is enough room in the design to add switches and IO if you want to do some Atari ST homebrewing and hacking.
I for example have programmed mine as an ultimate ripper and added the 'stealth' switch.
If you would like more information there are videos on YouTube: https://youtu.be/9ODUtvYLV-4