If I take a solid bottom board and flip it upside down to make it a cover/top entrance, would that work
or there's too much empty space between the "cover" and top of frames?
I have done this on Nucs when doing trap outs. When I needed to get the entrance closer to where the come and go. I had no problems with it.
If its reversable, try the 3/8 inch side. I think it will work fine as long as you make sure they dont run low on room. I find that when they run short on room they love to build burr comb on inner cover. If I give them plenty of room the inner cover stays clean.
Yes it will work:
http://www.bushfarms.com/beesmisc.htm#BottomBoardFeeder (http://www.bushfarms.com/beesmisc.htm#BottomBoardFeeder)
You'll get a bit of burr, but not too bad.
It's works just fine with the 3/8 side down, just make sure the top is sealed and the paint is thick. :-D
This is what I did on my hives
http://brooklyn55bees.blogspot.com/2010_06_01_archive.html (http://brooklyn55bees.blogspot.com/2010_06_01_archive.html)