Low or high density polyethylene or polypropylene would be my choice. Lexan is still not as tough and too stiff. All the flares I've seen have some detail as far as recessed screw locations etc. Unless you can eliminate those details and keep it a soft roundy shape, then you're a bit beyond borrowed oven elements or crude nichrome and shop vacs here. It will take a better than average system for good results.
Over 1/4 in. will require two sided heating but I'm positive I've never seen flares thicker than 3/16. When you add shape, it feels much surdier than a flat sheet. Start with 3/16 HDPE or poly, LDPE may be too soft?
Oven design will be critical for thicker sheets, The infra-red wavelength should come close to the absorption rate of the plastic. Don't guess here, use a proven oven design. You'll be dissappointed with less than 25 in. Hg, and 27+ would be better, but you'll still get some usability even at 20 in.
A/C compressors have lubrication and rust problems when used in an open system, but may work for a while if you can get them cheap. 4.5 CFM is a bit small for a 2 x 4 but can work OK with a tank. A good two stage would work well using a tank, then direct pump, but your Platen will need to be airtight, not sort of airtight. or the second stage will be inneffective.
I think its a good product idea and it can be done for sure with decent equipment but you'll probably have to spend more than its worth for a couple sets of flares. Entirely feasible if you need to make lots of them.
Warning: I sell plans for VF machines so I could be biased, but check them out if you don't have time to invent your own. http://www.build-stuff.com/1002plans_proto-form.htm