An Ender 3 sized CoolerMaster Masterbox Pro 5 RGB PSU cover.

This has been shortened slightly to fit the 220mm print size of my Ender 3, and has a slightly modified design to make it more 'printer friendly'. I printed this with a little cutout for a display and it fit perfectly, albeit a little tighter than the original.

I drilled out the hole for the retaining screw at the back of the case, but you could also use a heat set insert or leave it be - the fit is good enough that it shouldn't wiggle free even without the screw.