Also looking to understand the exact dimensions of the entrance, thinking about making one myself. I’ve seen a few online I like and since I’m doing 12+ of them, it would probably be easy and cheap.
No external feeder will eliminate the very real risk of making bees become robbers of adjacent hives with lots of fighting and the death of a large number of bees. The only feeder I recommend is the type that you take a frame out of the super and fit a frame sized feeder in. With robbing happening there will not be the amount of foraging happening so the loss of honey being produced, in fact more honey could be consumed for energy to fight than is being collected.
Marty, I would measure the entrance of one of the two hives you have and work from those measurements.
Cheers