Well, I may as well post my results here, since nobody answered.
I first tried heating plastic using a heat gun and also with a candle, both ended in failure since it wouldn't follow the complex angles involved in the shoulder piece. Both a drape and a negative mold. The plastic I used was from blister and clamshell packaging, easy enough to get a hold of. If the curve had only been in one dimension, it seemed like it might have worked. Even with pieces that started out in vaguely the right shape, they straightened out on heating.
After that, I broke down and bought casting supplies (expensive!). Both products from Smooth-On; Task 4, which is a 2-part resin that cures hard and preforms especially well in super thin pieces. And Mold Star 30, a 2-part platinum cure silicone that makes sturdy molds that have a minimum of shrinkage.
Platinum cure silicone is supposed to be rather tricky to work with, but I didn't have any problems. I simply used a non-drying modelling clay that had no sulphur in it (extremely important).
I took my first clean casting and have tooled it fairly extensively, thinning it down, shaping it more intricately, and adding the socket end of a ball-joint into the inside. Once I've gotten everything perfect, I will make a new mold of this piece and cast what I need. The old mold will be cut up and reused as mold filler.
The downsides of Task 4; First and I think foremost, is cure time. It takes forever. 16 Hours
in optimum conditions. Aka 23-4˚C 'Room Temperature'. My house is never that warm, except maybe in the summer. So usually it's more like 18-19 hours before you can take your casting out of its mold.
Second; Air bubbles. Holy crud. Maybe it's just me, but I'm getting an enormous amount of pinhole bubbles in a lot of my castings. Apoxie sculpt doesn't like going into pinholes without a fight, and there's no way I'd risk using something else that might have sulphur in it.
Mold Star 30; no complaints, other than the price. This stuff works like a dream, I haven't had a single problem with it. Even air bubbles haven't really been a problem.
Hope this helps somebody else out.