I am into vintage electronics and receivers and a recent purchase was missing this cover leaving dangerous voltages exposed. I designed this in On-shape using tutorials found on the YouTube channel Teaching Tech (thanks Michael). There are two version for different screw types. One is for countersunk flat top style screws, and this looks the best in my opinion. The second is for a button head or socket style screw. It works but just isn't a nice looking. 

I used M3x10 screws that were in my Prusa spare parts kit. Depending on your application, you can likely use M3x12 or 14. 

This is a quick print, and with PrusaSlicer .2mm layer height preset, it prints mostly solid in about 18 minutes.