personally, I would NOT use drywall behind your Mylar ... I was going to do that too, but then I remembered how sensitive drywall can be about moisture and humidity ... plus, then the mildew gets ahold ... I wound up using fairly cheap waferboard, a plywood-type material (avoid particle board, as it really sucks when wet, too) ...
glue doesnt work worth a damn on Mylar, so I just tore off small pieces of white duct-tape, and stapled thru that ... :smokin: