I would strongly urge folks not to use transparency film. If anyone has ever had really good focus at the EP and then stuck a low to mid grade filter on you noticed the focus slightly out. In the case of filters their optical quality is a fair bit better than transparency film. If you put one or more sheets in front of your scope the focus WITH the film in place my be perfect, but when you remove the film the focus will most likely be off if only slightly.
I would like to see a comparison of someone using a transparency made mask to achieve focus and then use a cut-out mask right after to see if this is indeed the case.

I would just hate to see folks use transparencies for masks, spend a couple of hours shooting, and find out that the focus was not as good as it could have been.

If I find some transparencies around here I'll give it a go here in the next week.