There is nothing complicated about upper entrance.
One simply cuts a notch in the rim of the cover. Cut it only to the 1/4" ply which is usualy set in a groove in the center, or there abouts, of the frame.
In old days, there was a smaller rim on one side and about twice as thick-a-rim on the other. One was for Summer use, other for Winter.
But, when one would flip it in winter mode, than entrance would be on top - which is wrong! Wrong for the simple reason that the air would than go through centre/feeding hole and out over the top and out. (Cross-draft)
Nowadays the inner cover is about 3/4" thick (not thick like the old ones) therefore is divided in 3 parts; 1/4" for plywood center and 1/4" for each rim. Simple.
One does not need to wory about location of the bees.
There is only one rule to remember.
Have your notch turned down at all times.
Have your upper entrance on the same side of hive as is the bottom entrance. Thus doing, no matter where the cluster locates, the air flow, so called chimney effect is always along the front wall of the hive and no cross-draft or any draft at all can reach your bees no matter what.
Thomas, throw the shims away, they are a pain in the but, more ways than one...
No matter who, what and how famous a beek may use them, they are no good.
For those who would want to get away from them, upper entrances, at other times of the year, (For the life off me I can't see why?) They can simply turn the cover upside down, or have a stick made, on a pin even, so it can be closed at a moments notice.
But, so affixed inner cover, closure on a pin is also a magnet for 'neighbouring kids' with ideas to close them and suffocate the hive... It has happened more than one would think...
Nation wide, upper entrance saves more hives than we can imagine. Often beeks don't even know that such a little cut in the top has saved their bacon... probably more than once a year - GUARANTEED !