Often the standard (push/pull flex tab) opening works fine and suits my needs; however, sometimes I wished I had a real simple second entrance with a removable plug... or a round hole that would work with a piece of queen excluder to tape over it... or even better a dial-an-opening entrance wheel/dial that could be rotated to various position/settings to vent, exclude queen, full open, etc.
PROBLEM: Didn't like trying to drill into corrugated sign-board plastic with standard spade bits and risk possible chatter, burring, or worse. Or trying to find my hole saw and running it backwards to make smooth hole.
SOLUTION: I had some of the 1 1/2" dia. yellow plastic nuc plugs that come from bee equipment supply companies (often used in the entrance of waxed cardboard nucs). Chainlink top line rail pipe is 1 3/8", but the coupler/connector had the O.D. that worked perfectly. Heated the connector for about 15 to 20 seconds and used it to make a nice smooth hole in the plastic nuc (opposite end from standard entrance. Now I can use either or both entrances. Queen exclude the round and close the standard push/pull entrance, then later open the standard entrance back up and/or plug the round entrance.
WARNING: Pipe is hot, do not touch with bare hands. I used pliers, but assume welding gloves would work.