The next thing you need is a carrier for the sheet plastic to hold it in place while you push the master through. Mine was scrap chipboard, a bit thicker than the master, cut on the scroll saw. The clearance needs to be enough to allow the master to go through reasonably easily whilst not catching.
Now take your sheet styrene and pin it to the carrier. My actual version used 60 thou sheet. Remember that there's a fair bit of stretch needed in the moulding process and the final item needs to be thick (and hence strong) enough to resist handling. I marked the outline of the hole in the carrier. If you do this it's easier to centre the master.
Here's the whole thing, with the master ready and the styrene sheet pinned in place.
Ready for action! Note cheap and nasty plastic card for a practice run.
Polystyrene starts to soften at about 70C, and at 95-100C reaches its glass transition (the point where it goes from hard and brittle to rubbery). You need it a bit above this but well below the melting point. Hot enough to stretch, cool enough to be stable. So, set your oven to slightly over 100C and let it warm up. I would recommend doing this when you're alone in the house or with prior agreement, if you take my meaning... I would advise very strongly AGAINST trying to do this over a naked flame. Quite apart from the fact that everything in the process is flamable, the chances of you managing to heat all the parts evenly are about nil. Now, domestic ovens aren't usually calibrated, and the temperature dials are not wholly accurate (you can take the man out of the GLP laboratory, but you can't take the GLP out of the man...), so get some small pieces of plastic. Heat one, assess the stretchiness, and adjust the oven up or down. Repeat until you're happy with the temperature.
Place the carrier with plastic on it in the oven, with the master. Wait a several minutes for everything to reach an even temperature. Then, USING GLOVES (I stress this!), remove the carrier, then quickly take the master and plunge it through the hole to stretch the sheet plastic to shape. Push hard enough to make sure the master is all the way through. Now LET IT TO COOL COMPLETELY WHILE HOLDING THE MASTER IN PLACE! Let everything get back to room temperature before you start mucking about with it.
Now you only get one go at this, and it might be that one or two go wrong. But it does work - and here's the proof! This one still has the excess sheet plactic in place. I had on practice with the chatty brown styrene, which didn't work, then this one, which did.
Moulded and firmly set.
Next I carefully (very carefully) cut around it at the point the master finished (marking it with a pencil or pen helps), and there you have your hull ready to add the details to.
Here's one I prepared earlier!
Now, just add the details! Doesn't sound like a lot of work if you say it quick, does it?