I made dis
Made it simple but somewhat filament saving. It comes in two versions - 5x6 and 6x6. 5x6 fits 256mm³ beds and uses 187g of PLA, 6x6 needs a bit more: 267x246mm and 233g of PLA.
The screw situation is a bit iffy, I measured my 3.5mm screws and the one I measured had a 6.6mm head, to I made the holes 7mm. But the screw that I measured had a slightly smaller head than all the other, so the screws barely fit.
I upload the f3d file so you can change that if you have the time and motivation.