The foil would be used instead of the black paper. Sadly dust is prone on mylar in a dry climate from static charges. A polonium brush like used in the darkroom may help.
Actually the best system I ever used for IR was the twinlens MF, with the IR filter on the taking lens you can still see to focus.

BTW what is a "Pro Grain" filter?
Are you familiar with WJs page and the Infrared mailinglist?
http://www.a1.nl/phomepag/markerink/mainpage.htm