I use an el cheapo heat gun from Harbor Freight, and made some frames that look like large, rectangular ping-pong paddles with a large hole in each (about three or four frames, 3/8" plywood or 1/4" pressboard). The hole size varies depending on how large the part being formed is, and the paddle frame is always slightly larger than the vacu box. Imagine picture frame with varying widths on the sides and having a handle.
The paddle approach allows you to focus heat on the plastic, while keeping your hand away from the heat source.
Attach a piece of plastic to the paddle (tape, clamps, etc.); heat the plastic with the heat gun until the plastic is sagging/drooping (it might take a few tries to learn the best "softness") then place the sagging plastic on the vacu-form box, and close the vent. If necessary, continue to heat. You can do multiple pieces, and they would be held in place by the vacuum of the box with the gate open. If the parts are too small for this, then use double-sided masking tape, as in that used to replace grips on golf clubs.
After the plastic has cooled, remove it from the frame and trim etc. Windex can be used to accelerate the cooling process...the evaporating Ammonia helps a lot.
I used to make the buck out of wood, lay the plastic over it, and heat it until the vacuum took over. I had to use wood, because heating plastic on top of a metal buck required that the buck reach a very high temperature, often leading to less-than-desirable results. A friend of mine built a similar box and shared his approach described above.
Bill? Does that answer your question, or did I miss something?