A Generac whole house generator has 4 lifting holes near the base. These make a very nice home for critters (until the generator starts). Unfortunately Generac does not supply hole plugs. This design covers the hole very tightly. I used it for a 16kW generator, but I believe it will fit other sizes also. Let us know if you use it on other sizes.
I found I got the best print by printing it on it's side. This minimized the supports needed, and the front printed very smooth.
I printed on an Ender 3 Pro using PLA, 0.12 layer height, and supports on. It required a little filing/sanding to smooth the area where the supports were, but then it fit snuggly in the hole.