LOL, I was just looking at this stuff yesterday. . . gotta love synchronicity!'glassboard' -- it is made in 'brilliant white' is glossy, pebbled finish for excellent light reflection from & too all angles. This is the stuff that now covers the walls of commercial kitchens $25.00 for a 4'X8' sheet at Home Depot. go to Home Depot w/ all your measurements & they'll cut it for you too.
I think the B&W poly is better than mylar for several reasons including reflective, workability and maintainability! That mylar crinkles up easily and if you wash it it will actually reduce the reflective properties of the film!Mylar all the way! Get the 2 millimeter stuff the 1 millimeter rips too easily and is hard to work with. The 1 mil. is not useful for anything but covering a perfectly flat wall. Second best choice would be to paint the walls with flat white paint or cover them in "panda poly" (white on one side, black on the other.) Personally, I use mylar.