This is a foot for the bottom of a 1515 aluminum extrusion rail or equivalent. I recommend also using a felt pad on the bottom, especially if you have hardwood floors. The screw holes are M5.
I've also included a modifier mesh covering the bottom platform and the area around the holes. You can load that into your slicer and designate those areas as 100% infill if you're paranoid about strength.
Any PETG, ABS, or similar
100% infill in the areas covered by the modifier mesh