Opportunity is missed by most people because it is dressed in overalls and looks like work. - Thomas Edison
Check these folks out..... "www.foamorder.com" they are located in San Francisco, but they have brick and mortar stores in several places in the Bay Area.I'd recommend a sheet of the 3mil neopreme with nylon backing on one side. It's what I have on my exposure unit and it works well.
That doesn't mean that facing on both sides won't work. Anyone else have one like that?My other source for various materials like this also currently has the neoprene foam with facing on both sides.Their stock changes regularly.
It sucked everything down tight, but when foaming around the tri-loc it doesn't want to get in every nook and cranny.
As long as there is no space between the film and the emulsion, you should be alright. Myself, we use pond liners from Home Depot. $40.00 worth was enough to 2 blankets, and our exposure unit is like 55 x 40.Steve