Those will be hard for them to see given their poor eyesight, that's good, but seems to me the entry hole is kinda big, maybe a 2 liter soda bottle cut back so the entry is like 1/2 that size? You can even cut them to leave "Fingers" to be bent over to block re-entry, but allow egress.
Holes are about 1 1/2" in diameter. It was a certain kind of iced tea bottle here that looked like it would do. Nothing seemed to be the perfect size, but I figured 1 1/2" would be ok.
I get what you mean about fingers. Makes good sense. I thought about taping a ziplock bag around the end so that getting in would be impossible but they would just slide out.
I havent found the perfect one yet, but getting closer to figuring out the ideal shape, size and orientation.
What diameter hole do you think would be best for the little brown bats?