3mm. thumbnut PETG

If you want to use the PSU holder for the PRUSA mini enclosure from their printables site:

https://www.printables.com/model/936064-prusa-mini-enclosure-psu-mounts

but do not have the filteroption with the necessary thumb nuts, here's a 3D print version of the nuts that work real good.
Print three of these with the widest part down on the plate, no supports nor brim necessary.