You will need the following:
3: Closet Flange Spacer Rings (Home Depot has "Raise A Ring" from Sioux Chief Mfg. (model 886-RQ), but others might work. Some other types don't have as many holes and might benefit from drilling a few extra.)
4: 1/4" x 4" hex bolts
4: 1/4" x 3/4" to 1" machine screws with countersunk head
8: 1/4" hex nuts
1: 1 1/4" x 1/2" PVC bushing (should have a flange on one end to serve as a good gluing surface, the other end should roughly fit your vacuum cleaner)
1: 4" PVC clean out cover plate (has a small hole in the center that will be drilled out to more or less match the inner diameter of the bushing). A 5" clean out cover plate also works and is a little easier to glue (it overlaps more). You can substitute anything flat made of PVC that covers the flange opening.
1: 18" of rubber weather stripping (I used Frost King EPDM Rubber Weatherseal 5/16" wide, 1/4" thick).
1: Can of PVC cement (I used Weld On 790 Multi-purpose)
The cost of these parts (excluding the weather stripping and cement) is about $16.
Step 1: Drill Hole
Step 2: Glue Parts
I've also included pictures of a base made with a 5" cover plate.
Step 3: Finish Base Assembly
Depending on your vacuum cleaner and specific bushings, you may benefit from constructing some sort of gasket or spacer to ensure a snugger fit. I used some ShapeLok plastic to make the inner diameter of the bushing more closely match my vacuum cleaner tube.
Insert the 1/4" x 4" bolts into 4 of the 8 holes in the flange extender, and attach with nuts so they form the vertical guide rails that the carriage will slide down on.
That is pretty much all there is to it. Now you just need to mount a plastic sheet between the two flange extenders that serve as the carriage, and it is ready to use.
Step 4: Mount Plastic in Carriage
Sandwich the plastic between the two remaining flange extenders and clamp them together using the 4 1/4" x 2" bolts and nuts. Be sure to use a consistent set of holes so the carriage can slide down the guide bolts.
Step 5: Use the Vacuformer
The carriage should be able to remain above the object simply by friction between the bolt threads and the edges of the carriage holes. Place the base on the end of a vacuum cleaner hose and then use a heat gun to soften the plastic sheet in the carriage. I've used a Wagner HT3500 on high at 1150 Fahrenheit but I'm sure other settings or other heat guns can work fine. Once the plastic looks soft (it should be smooth and droopy, and for some plastics the color might change), turn on the vacuum and lower the carriage down the guide rails until it contacts the rubber gasket on the base. At that point the vacuum will almost instantaneously suck all the air out and you are done.