8T,
I've never used 'Magic Shrimp Foil' and I've tied thousands of Czech nymphs.
Scud back, shrimp back, any stretchable plastic, even stuff cut from a plastic bag will work just fine. What I use is the clear stretch 'elastic' that they sell in the yardage stores for use in bathing suits. I split it in half and color it with a magic marker. For the under 'ribbing' I use Sulky tinsel (sold in the same place) that's basically thin colored plastic mylar type stuff on a spool. Comes in a good range of colors, but I use mostly greens, oranges, and pearl. Thin Flashabou or even krystal flash will also work.
Don't get too anal about the 'exact' patterns on these things. What I found to be 'critical' are: the use of lots of weight. Wrap the entire hook shank and double over it on the larger sizes. Keep colors simple. I tie olive, tan, and yellow only. Make sure that you use a dubbing with some sparkle to it. I looks just right through the plastic, giving the nymphs a subdued but unmistakable glimmer to them.
I tried some with and without the obvious color change fom the body to the head, and couldn't tell any difference to the fish in use. Now I just use a lighter color dubbing for the whole thing, and use a marker to darken it at the head before I warp the under rib.
I don't bother with legs. I used to pick out some dubbing at that point, but gave it up as the fish don't seem to care.
The key to this is the technique, not the flies. It works with heavily weighted hare's ears, prince nymphs, copper johns, etc.. I often use a really heavy stonefly nymph as the bottom fly (I mean REALLY heavy-you could cast it on a spinning rod). Use flies you have confidence in for your waters.
Good Luck!
Buddy
It Just Doesn't Matter....