Hi Allen
I use 4 mm acrylic sheet, double glazed 10 mm gap
It works, but not as well as glass as it blocks some frequencies.
When I first made it I had three sheets and two 10 mm gaps, but that barely
worked at all.
I have plans to make a new top that is double glazed glass and is much
larger than the box.
This will allow sunlight into the box at a wider number of angles and will
thus give more time per day.
Regards From:- Dave Cushman, G8MZY
Beekeeping and Bee Breeding, http://website.lineone.net/~dave.cushman
IBList Archives, http://website.lineone.net/~d.cushman