RE: Vacuum forming questions
We always used MDF (at the local home improvement store) for building molds. It comes in 1/2" - 1" sheets. You glue and stack them to the height you want and cut the mold. The mold box can be made of two MDF sheets also with a plenum between the two (built using 1/2" thick strips and double sided tape). Put a pipe flange on the bottom plate for your vac source.
The MDF sands smooth if you work it down with 220grit, 320grit, 400grit then spray silicon spray on the mold liberally. Wait 2 minutes and sand the silicon coated wood HARD with 400 or 600 grit. It'll leave a surface that should be good for up to about 50 pulls. You can silicon and sand again if you develop a problem area on the mold.
0.30 - 0.40 PETG should work VERY well.
Drill 1/16" vacuum holes around the mold into the plenum and in cavities in the mold for a excellent pull.
I can draw something up if your interested.