It's hard to tell from the pic, but the opening looks to be about an inch tall. If so, that's much taller than I would have made it.
It's exactly the same size as an bottom board entrance reducer that way I can interchange them in various size openings with the bars i already have.
Much easier to just use the solid bottom board like a migratory top and the reducer is still usable--works for me. I'd like to see your burr comb situation about a month or so after installation. Good, innovative, design, just a tad over done.