Fly tying shops are selling a promising new plastic sheet material named "UV Chewy Skin" for $5.50 or so per 2" x 5" rectangle.
The Troutfitters shop in Bozeman MT can't keep it on the racks. Local tiers cut it into variable width strips to make San Juan-like worms and Crayfish and scud backs. The one big drawback is the price. It sure isn't cheap.
For about $3.50 I can buy a sack full of bass fisherman's skirted tubes that represent 2 or 3 times that many square inches of gooey-soft multi-colored sheet material. This stonelfy nymph was snipped out of a Berkeley Pumkinseed Powertube. Works too. Damn good fly. I used it to tie both of the flies below.
Recipe:
body bottom: open cell mattress foam....not much. Just a thin sliver
body top: a long triangle snipped out of a power tube
legs: rubber legs
hook: scud hook
glue: CA glue
You would need two needles. One is any wide-eyed sewing needle whose eye has been made wider. Use a cigarette lighter, pliers and another needle to widen the eye. Then you can use it to sew rubberlegs into the body. The other needle is an ultra-thin #13 beading needle. Put that in the vise horizontally and then make the body, without any hook at first. Skewer the foam onto the needle. Sandwich two end-to-end rubberlegs between the foam and the rubber skin. Lash it onto the needle, segmenting the body as you go. Whip finish up front. Slide the modular body off the beading needle. Now put a scud hook in the vise upside down. Wrap the shank. Lash on the body, so the hook points up. Sew in two more rubberlegs. Now soak the top of the fly with a drop of CA glue, so the hook won't spin.
CA glue adheres to some plastics and not at all to others. CA glue does make a ferocious grip to plastic tubing material, for what ever reason.
You can also sandwich a bit of flattened lead into the thorax area, which forces the hook to always ride up.