This is how I've done my top entrances (shown upside down):
I started out with long hives, which were made so as to accept three 8 frame medium supers, and these inner covers were protected by a sheet of galvanized corrugated roofing. The top entrances worked very well for the long hives, and were much more flexible than bottom entrances. For example two colonies could be housed in the same long hive.
When I put a couple of colonies in conventional Langstroth equipment, I made the boards shown above. The entrances are just 3/8" slots routed out of the bottom side of 3/4" ply. The entrance size can be adjusted with bits of scrap 3/8" stock and the feeding hole can be used with an empty hive body. I use migratory covers over these entrance boards, if not feeding.
I'm a beginner, so I may realize that I've been wrong all along, but so far the bees seem to be fine with these entrances.