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Stress on the film is because people tend to wrap it over the end of the lens and pull it back. I got a nice flat result by buying a "lens hood" which was basically a conical step up ring made of aluminium. I ran tarzans grip around the end of the ring and sat it glue down in the corner of the film to set. I left it alone for a few days, trimmed around 5mm from the ring, a little more glue and the trimming was folded back (not stretched) to the sides. When set a little black tape covered the sides to neaten it up and ensure no gaps for light leaks (though I was careful to make sure the glue on the front had no gaps, it completely sealed the end). Only cost me a couple of bucks for the ring. I use it regularly, film stays nice and flat (slightly slack at times, never tight though...thermal expansion of the aluminium I guess).
A few more bucks for a container to protect it (I used foam pads on the inside of the container whichs "holds" my filter suspended firmly so the film never touches anything). For my scope I found something suitable in the plumbing section of the local hardware store.
Probably the better solution would be two low-profile UV filters with a very carefully trimmed to size piece of solar film sitting between them. It would stay protected from dust and finger prints. Use a pair that are larger than the camera lens and use a low-profile step-down ring to ensure any light leakage at the edge of the film is blocked. Or paint a black border, or find a rubber O ring...doesn't much matter as long as there isn't any stray light coming in from the edges ruining the shot.
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