At 3DUPfitters we sell a Prusa i3 MK3S enclosure, but many of our customers use that enclosure for other printers. That's great, except the enclosure comes with a cutout on the right to supply air to the power supply.
Plus some Prusa owners don't care about power supply longevity and just want to get the insides as hot as possible.
For them this handy cover completely seals off the PSU vent. The only tricky thing is you really have to make it from ABS or PETG in order to withstand the heat and not warp.
Note that the picture shows a customized version we made that includes a rubber grommet. This basic design was made to be universal and so has no hole for a grommet.
This will fit any of the Prusa enclosures sold in April 2019 or later. Earlier versions had a slightly larger PSU vent.