They're really not that hard. You have to decide how large a part you wish to copy, this will determine the dimensions of your box, and that is all the machine is! You'll need to make a box with 4 sides and a solid bottom, with the seams sealed to make them airtight. I built one from 3/4" x 3" pine from the lumber store, with a 1/4" masonite bottom, all glued and screwed together. One side was drilled out with a hole saw to accept a fitting for my small shop vac. The "stage", or top, was masonite perforated board.
You need to make a frame to hold your plastic sheet, again I used 1/4" masonite and made two open rectangles, kinda like picture frames, that would trap a sheet of plastic between them. I used flat head screws and wing nuts to clamp them together, with the screw heads recessed flush with the masonite. Make a "gasket" with some self-stick foam insulation over the screw heads, and that is your vac machine!
I heat the plastic with a heat gun, you could use an oven, whatever is convenient and easy to control. With your master on the stage, soften the plastic, turn on your vacuum, then bring the frame with the softened plastic down onto the stage and you will have your vac-formed part.