What kind of mats do you put on top of the AJ traps to keep the bees from covering with propolis and wax?

I purchased some AJ's Beetle traps from Dadant earlier this year to be ready for any SHB outbreaks here in Central New York as I had a couple of cut-out hives wiped out from SHB here last year. I just relocated a new cutout to my bee yard and saw a two SHB run off the bottom board when I check it last weekend. I want to put a few of the traps in, but don't know what kind of mat to use.

Any additional tips, suggestions, tricks would be appreciated.

-Eric