This is a quick bracket I designed in Fusion360 to mount an Anker USB Power Brick underneath a table.

The screw mount holes have a 5mm diameter with a chamfer to 6mm so my 6.2mm diameter screw heads will sit flat. I've included the Fusion360 file so you can edit this for your screws if needed. You could also get rid of the screw holes entirely and use something like command strips to mount this on a surface.

The part has a very low tolerance of about 1mm. If your printer cannot print with accuracy of at least 0.2 mm, you may want to edit the file to create more tolerance.