This is a wall or bench mounting plate for 20x20 v-slot aluminum extrusion. Has a v-shaped wedge to help with centering/stability. To fasten to your extrusion use M3 screws and the usual hardware. I recently discovered these nifty roll-in T-nut pieces which you can insert into the channel without sliding in from the end. The screw hole is recessed so the head won't scratch the wall/bench. When used with the T-nuts above an M3x8mm cap screw fits perfectly.
The other holes are also recessed but are a bit larger (around M5) so you can use standard wall fasteners.
Prints easily with minimal support (two tiny supports underneath the recessed screw holes).
Anything you want. I used 0.3mm