Filters shift the focus because they are placed in the converging/diverging light path. The light entering the telescope is parallel and adding a flat plane across the entrance should not shift the focus. The low optical quality of a plastic sheet may fuzz things a bit, but it shouldn't shift the focus. With respect to the focussing mask I suspect it will make the lines a tad fuzzy, but it shouldn't effect how the mask works or the relative placement of the reference lines. I might give this a try for some of my smaller telescopes where the line spacing makes cutting a mask by hand out of foam core difficult.