The next few ovens that I built were a little more advanced. I experimented with using cement board and casting concrete for the body of the ovens.
I added reflectors made out of aluminum flashing, so that the heat was directed up to the plastic (and not into the oven walls). This made the process much faster and wasted less heat.
I also changed how I made the heating elements out of Kanthal. Instead of coiling the wire, I used straight strips that were connected to threaded rod and suspended on springs. This meant that when the Kanthal heated up, it would not sag and touch the reflector (which I connected to ground as a precaution).
I also added a Raspberry Pi with a thermocouple and webcam, and connected the heating elements, the air compressor and a solenoid valve up to a bank of relays. This enabled me to operate the oven from the keyboard. My intention was to use OpenCV and the webcam to blow the bubble automatically, but I did not complete that. I realized that it did not make sense to automate that part of the process before I developed a better way to load and unload the plastic.