I don't have a top bar, but I did add a slatted bottom board. At last inspection heaps of bees were hanging out under it. but, No burr comb was built there. Seems they enjoy the space when the hive is busy and full of bees. There are posts about this idea of a slatted bottom board. Good luck.
"... and why the boiling sea is hot, and whether pigs have wings."
Thanks, Grieth, I don't know what a "slatted bottom board" is, but it's good to know there was no burr comb and their liking the additional space in which to hang out.