I've been eying vacuum forming for a long time.
It's a quite interesting technique although it only suits some particular projects.
I'd love to use it for some cases (raspberry Pi perhaps) and other ideas but...
I've tried with some pet bottles and that plastic, simply shrinks.
Saw someone who used PEHD (the same from plastic bags) and it also shrinks.
I've found PP drinking cups but, no luck at all, as you can see: