I was pleasantly surprised by how well HDPE 'fused' - by this I mean, with almost all Vac Forming, you can get 'webbing' where some material stretches and distorts. It can of course create imperfect moulds, but HDPE (perhaps because it is so soft), stays warm for a good wile, and melts back together nicely, leaving no clear trace in the mould of a weld. 

Of course if I were casting a super high detail thin resin, this might be noticed, but certainly for anything like Plaster, or a resin you intend to finish-up/polish anyway, this really is not a problem.

If you would like to heat the plastic up quicker, you can create a shroud using the 1" / 25mm metal as shown. But if you don't have this, I found a biscuit tin worked, if you have some snips and sandpaper to cut and smooth off...

The slightly more rugged metal strip version.