Step 3: Construction

Picture of Construction
Most any soft flexible plastic plate will work however, it should be a thicker reusable plate (2-3mm thick). Thin disposable plates are not structural enough
The type of plate used here is from a family of plastics known as Polyolefins.  Common grades are Plopropylene (PP) or High Density Polyethylene(HDPE).  (HDPE is the type of plastic used in milk jugs.)
Olefins are a soft, almost waxey feeling plastic. (Ref. Rubbermaid or Tupperware containers)  
Stiffer acrilic type plastic plates, which are more brittle, probably will not work.  They likely will will not form as well when heated.

1. Drill a hole in the plate.  
The starter hole is much smaller than the required finished diameter. The "Volcano" shape is formed during the process of stretching the starter hole.  Try various starter holes sizes to get a taller volcano draw depth (~3/4" dia for a plate to be used with a glass bottle)  
Experiment and customize with different hole sizes to accommodate different types of containers

2. Heat the center of the plate.
A hair dryer works well (only because I don't have a heat gun)  but, it could have used the higher tempatures of a heat gun to make the proceess faster.

The use of a torch to heat the plate is not recommended.  Too much heat, too fast.  You only want to soften the plastic to make it pliable.  A torch will take it to molten & scorched in no time.
If the plastic becomes too hot and approaches moltan rather than just softened, there is a good chance the plate will warp beyond use.  The hair dryer never got it hot enough to take it to that point however, becasue of the lower heat output, it took several minutes to sufficently soften the center of the plate.

3. Stretch the hole to form the "Volcano"
While continuing to heat the plate, push it down over a mandrel to stretch and form the volcano shape.  I used a glass long neck bottle as a mandrel.  For different size holes I used various long neck glass condiment bottles to stretch the hole and for one I even used a tapered shot glass.
- There will be some shrinkage as the plastic cools.  Leave the hot plate on the bottle as it cools to minimize this.

4. Finish the hole edge
While a hair dryer / heat gun was used to soften the plastic plate for forming, a torch can be use for a finishing touch.
Carefully, and quickly, pass a torch flame over the raw edge of the volcano rim.  This will deburr, and round the hard square edges of the cut hole. The torch will add a finished soft edge to the hole but, care must be taken with the torch or the extreme heat will, melt, warp or make other areas of the plastic shiny.