Very good D.I.Y. vids those panaba453
I agree that the way you do your shells i.e. heat up the Polycarb seperately then press it & vac it on to the mould, will give the buck a longer life as you are not directly heating up the buck initially. And your master is a very large, thick buck which also helps. But a lot of commercial vac-formers, for example like mine, place the buck underneath the clamped sheet material with then heat over that. Depending on the sheet material & thickness used the bucks can get awful hot very quickly, and then are cooled down very quickly on de-mould. Basic plaster of paris dosent like this very much, especially if its a small buck.
And yes, there are different types of plaster and various ways of reinforcing them for moulds, some quite strong that could last for a good while with your method. But If i need to do something like that i just go straight to the works-every-time polyurethane resin cast in a vacuum chamber.... the buck comes out with no imperfections, will last for ever and has a high heat deflection temperature range... Not exactly D.I.Y. though i admit
I have to say though, anyone contemplating doing their own D.I.Y. bodies could follow your vids and get a very, very good result