One of the bird box entrance reducers I installed nigh onto a decade ago is still on duty, although downy woodpeckers definitely want a larger hole:
Another reducer had gone missing over the years, so I made one from a length of PVC pipe:
It started as 1-½ PVC pipe, 1-⅞ inch actual OD and should fit into a 1-½ hole, so I measured 1.5 × 3.15 around the circumference, bandsawed out the excess, draped it over a 1-½ Forstner bit, toasted it with a heat gun, and squashed it so it’s just a little bit bigger than the (enlarged!) hole in the box.
Now the entrance is 1-¼ (-ish), just like it should be:
The oblong hole required advanced manufacturing techniques:
The front face should be too slick for larger birds and the little ones will zip right into the hole:
The two starlings who’d been evaluating the box seem to have moved on; we doubt they’re now homeless.
The OpenSCAD source code as a GitHub Gist: