I was looking at pressure for the top but in my case went to vacuum. With pressure what scared me was you need an air tight cover over the system and applying positive pressure I was looking at a bomb. Especially when it gets larger. The guy's can comment on this one as some are more knowledgeable than me.
With pressure I believe you will also cool the material very quick where with vacuum you are taking it out. For molds I make mine from foam but I also cover it with epoxy and glass. The best I find to make a mold is to make a shell and fill it with Polester resin and sand mixed together for weight and strength. The problem with large sizes when using the resin mix is that it will shrink so has to be done a bit at a time.Styro I find will not take much more than 7 or 8 hg before crushing so by itself it is good for making a mold for the other processes.
I made a machine for myself for doing canopies and items for my RC hobby. It will pull down a sheet 24 x 16" and is self contained. After I was given a Di Arco machine that is smaller but has a pump/vacuum compresser with tanks for each side and with a Pyrex heating element that swings over the top.
If I can give any help let us know.
John