I watched Matt using his homebrew vacuum former and he kindly explained what he was doing.
At the top of the device is an electric heater which heats a sheet of plastic until it’s pliable.
Under the heated plastic is the mold.
Once the plastic is soft enough, Matt drops it over the mold and turns on the vacuum pump (the guts of a vacuum cleaner are in the box under the mold.
The pump draws air out through a perforated sheet, sucking the plastic right up against the mold. When the plastic cools, he has another piece of his Ghostbusters proton pack.