For those who have a mini-rack, and were wondering what to do about a lid.. had you noticed that it's about the size of a 4x4 gridfinity base? This is a quick frame that creates a 4x4 gridfinity refined base mount as the lid of an 8" mini rack - so you can then add whatever monitors or extra widgets - or just use it as more storage space. 

Gridfinity refined has the option to mount things using screw-mounts - so avoids the magnets - which I've done just as a precaution for my server rack (no idea if the magnets would have caused an issue - but I didn't want to find out)

The mounting holes are M3 clearance btw, and spaced at 179.5 mm - which fitted my rack perfectly (with a bit of tolerance). The design uses the same piece, printed 4 times. 

Enjoy guys - a bit niche this one - but I hope it's useful to someone. 

Kudos to Grizzie for the gridfinity refined variant.