My old iMac 24" 2009 was too slow for Catalina and a new iMac M1 too expensive so I bought myself the cheapest Mac Mini M1 (8GB, 256GB) and a Samsung T7 1TB external SSD as well as an AliExpress TFT conversion kit and some other parts to convert my 24" iMac to a full-hd HDMI monitor with sound.
My Mac Mini is set up using the internal SSD for apps and general stuf, my user folder is on the Samsung T7. I get an average speed of some 660MBs to the T7 so good enough.
To mount the Mac Mini on the back of the iMac I needed a holder and a way to mount this holder so I designed a bracket that clicks on the iMac stand and allows the mac mini holder to be attached. The set is very rigid. Mac Mini power button can be reached by passing one's hand under the iMac to the right of the stand.

Full print with 4x4x4 walls/top/bottom and 38% infil came to about 48g in weight.

Glue in lettering and spade. Star may need minor sanding but should push fit into the space tightly and not need anything else to secure it.