This is a quick and dirty modification of an existing Pi 4b case:
https://www.thingiverse.com/thing:3852672
to fit on the mk3s frame and fit under this light bar (the dual light):
https://www.prusaprinters.org/prints/3267-led-light-bar-prusa-i3-mk2mk3
I used the version designed for 3mm screws, but had to drill out the holes anyway. The socket head cap screws don't sit flush - fyi.
See the thingiverse link for information about mounting the fan or other information.
The author hasn't provided the model origin yet.