My area of expertise this, been a professional modelmaker for years. As you can probably guess from what follows I am looking at producing shells myself.
Pro body makers carve the body design in clay,(which is easy to work with by hand) or modelling board (easier to machine). When the pattern is finished a mould is made from the pattern in silicone, then the mould is made in tooling resin, which can withstand the temperatures of the vacformer. Total cost of the materials for this is in the region of £300 per pattern. For a production run tooling resin (at £50 per kilo!) is the only option that will last.
Bodies are vac formed in polycarbonate, if anyone wants any 1mm polycarbonate let me know, I usually sell it in A4 size pieces but can supply bigger sheets. Vacforming polycarbonate at home isn't practical, apart from needing to build a vacformer you need to dry out the polycarbonate before you can mould it.
Oasis foam will just melt if you try vacforming over it. Epoxy resin will also attack it.
The best material for making a pattern from is modelling board. Softer boards are easier to carve but harder ones last longer. They have no grain, so gives a great surface finish. Vacforming over it will distort the pattern over time, but perfect for prototyping. I get mine from here
http://ccgi.johnburn.co.uk/products.php?broId=17
An alternative is jelutong timber. Easy to carve and relatively stable with no knots and a very close grain. It still has a grain so the surface finish will always have a texture when formed over, unless you use a filler over the top of it. Very soft wood, easy to dent accidentally. Also distorts with repeated use as a former. I get mine from Arnold Laver at Mosbrough
http://www.timberworld.co.uk/sheffield.asp
Also available in small quantities from EMA
http://www.ema-models.co.uk/shop/cat...e-LF.html#LF11
Once you have made your pattern you will need to do some work before it can be vacformed. It will need drilling with a 1mm drill in all the low points to get air out of between the pattern and the plastic as it is moulded. Look at any bodyshell and you should be able to find small dots in the body, this is where it has been drilled to vent it. It will then need fitting to a mounting board, the design of this depends on that machine it is vacformed on and you will need to discuss this with whoever does your vacforming.
BTW Attached are photos of a recent project of mine, shows what can be done with jeltong, filler and urethane resin.
