In the past I've tried vacuforming at home without much success. I had built a very small vacuformer with the usual plethora of holes on top and couldn't get anything good out of it. For this project I tried using the single hole platen design with my shop vac, as described by drcrash in this Instructable. This table actually works very well, and I was able to get good pulls of all of the armor pieces with some practice getting the timing and such right. I won't bother posting pictures of the table, just check out the instructable to get the idea.
That said, there's some wisdom to be gained here, as I screwed up a bunch of stuff! First, the bucks (masters) for the chest armor parts were barely strong enough to stand up under the pressure of the vacuum and hot plastic. The front piece faired the worst, I pretty much ruined the master with my first pull. Rather than starting over, I filled the plastic shell with plaster to get a nice solid buck and try again. I had the bright idea that I should skim coat the inside first with a coat of Durhams Rock Hard Water putty, since it's stronger than regular plaster. I reinforced this with some fiberglass cloth then poured plaster over it to fill the rest. Well, when I removed the plastic shell a bunch of the water putty detached from the plaster and I ended up with extra repair work. Anyway, I was able to fill and repair all the dents and get the new buck into a presentable state. This one is solid and should last if I decide to make any more. One more tip - I was getting some striations in the plastic, which turned out to be cooling stretch marks, as it cooled quickly when being sucked over the master. The solution was to pre-heat the plaster buck in the oven on warm for a bit, and apply some mold release to the surface so the plastic slid over more easily and cooled more slowly. The end result is worth the work, these armor parts came out great!
Here's what a couple of crappy pulls look like. The creases are known as webbing, and can happen when your parts are aligned in such a way that the plastic pinches up before it can be drawn down over the master. These were solved by fiddling with the angle of the parts on the platen. I managed to get all the parts out of a single 4'x8' sheet of .06" styrene, about a 50% waste ratio.
Behold a picture of my masterfully executed shoulder strap buck! This simple piece was made from pieces of balsa glued together, the only part of this costume that was easy. ;) After forming I headed them up with a heat gun and bent them to fit over my shoulders. Careful using the heat gun on styrene! You can over heat it very easily. Below you can see the vacuform masters set up and some better pulls. Note the angle at which I placed the shoulder bell master to get rid of the webbing. In these pics I'm using a cooling rack to raise the plastic and allow air flow to the vacuum, just an experiment to see what worked best (drcrash recommended folded up window screen). The cooling rack worked fine, but no better I think than just raising the bucks on other objects.
That about covers the armor construction. Time to move on to the soft parts, mostly a shopping extravaganza.