Re: 1.5" Shim with Telescoping Cover for Top Entrance?
The tele cover is intended to use in conjunction with an inner cover. The inner cover acts as a roof from the perspective of bee space, and the tele provides a weathertight cover that should not blow off by virtue of the overhanging boards that overlap the top hive body.
A migratory cover with shims provides the same beespace function as the inner cover while not utilizing the tele cover at all. That would be why rocks or bricks sit on the migratory covers.
The problem with using a tele cover by itself is that if the bees propolize it in place it can be very difficult to open the hive because the overhanging boards prevent a hive tool from getting access to the junction of the cover and top box.
So for a better answer to your question tell us what you have in mind for the inner cover, or are you thinking of eliminating it?
--- Victor Hugo - "Common sense is in spite of, not the result of, education.”