Zulkit 5.9 x 3.9 x 2.8 inch Enclosure Interior Mounting Plate

This plate has a 5mm grid of 2.9mm holes, and two screw holes for the M4 screws that go in the M4-threaded inserts inside the case.

The holes should be relatively the right size for just threading M3 screws right into the plastic. I doubt you could repeatedly thread in and out, as always. I'd go with PETG or something tougher. It's an outdoor enclosure, PLA will easily warp from the heat inside the case if you live anywhere where it gets warm.

OpenSCAD file included, so if you want to use this with other enclosures, you might be able to adapt it for them.

Enclosure: https://www.amazon.com/gp/product/B08KY7VK8W