The birdfeeder I bought at Walmart became quite popular amongst the birds in my backyard.

The only problem were grackles, who scooped out pretty much everything trying to get the black sunflower seeds. The feeder was emptied in three days, most of the grain ended on the ground feeding mice and rats, no good.

I added ports to prevent that spoilage, seems to be working.

The feeder cup diameter is 96 mm at the lowest/narrowest bottom opening.

UPD: Added two more other models, the cupola works better.
UPD2: Added a much bigger cupola model, had to cut the opening in the feeder wider, but that one works much better. Almost no spillage, doves are struggling to sit there, so they are not staying long enough to scare off smaller birds. Cardinals, sparrows, finches seem to be comfortable enough.