As you can see from my sketches below the 3/16" space above the top bars of frames has been a concern of mine for over 14 years. Why? Primarily due to notices accumulations of hive beetles which scurry when I left off the cover to my hives. Hive beetles shouldn't be able to escape the bees. Often after removing the cover, the bees would quickly begin chasing the beetles which they weren't able to reach until I lifted the cover.
I guess there are two ways to approach the 3/16" space. Down here in South Florida we do not use inner covers. In the absence of inner cover and the common practice of using 3/4" plywood or migratory covers as top covers this leaves a 3/16" space. A bit too tight for most bees to squeeze into to chase pests or for whatever reason they may need access.
One approach is create a larger space... just a slight increase to cause it to be 1/4" or 5/16", but really not 3/8" otherwise more burr comb and frames sticking to top cover. This could be accomplished in hive cover design, spacer shims, etc. (not going to think about it now... another day).
A second approach is to reduce the space to the point hive beetles cannot access it. This is where I will focus today.
Having also used top bar hives (KTBH & TBH) from time-to-time I noticed that my weather/rain cover actually fit tightly against the top bars, not to mention that bees did not access the area above the top bars due to the design of the hive (no spaces between the tight-fitting top bars). Why not have such a cover or inner panel to fit tightly on top bars and eliminate the beetle space?
I am going to test 3 of my hives using the 3/16" panel. I used panel bead board which I found to be reasonably stable on top bar hives... plus I had some on hand. I guess you could use other panels... or even Plexiglas.
Goal is to keep top bars free of hive beetles and forcing any hive beetles to expose themselves where my bees can trap/jail (propolis them in) them. We'll see how it goes.... or if I need to make modifications, improvements, re-design, or abandon this for another decade.