I noticed that the PCIe slot cover on my NZXT H510 Flow PC Case did not sit flush with the case and sketched up some printable covers that fit better.

The .3mf file contains four variations of the cover. There is a solid cover and three patterned/mesh covers that let air through. These patterned covers are designed to have no top or bottom layers, where the infill itself creates the patterns. You can simply open the .3mf file in your slicer and hide/delete the covers you don't want.

I printed my cover in PLA, it never gets hot enough for it to become soft.