Have you tried hot water?
Find a cylinder of about the right diameter, or a bit tighter, such as an empty can or glass bottle. Tape your plasticard to it, probably 20 thou max as you have to force it round. Boil the kettle and fill a jug with the hot water, then dip your cylinder in (if it doesn't fit in a jug just pour the hot water over it). After you've taken it out and let it cool, remove the tape - your plastic will stay (nearly) at the same curvature as the cylinder.
Much easier than vacuum forming and only takes a few moments.