There are always dangers when working with mains voltage, propane and plastic fumes. Take precautions when building and operating the oven. Do not leave the oven unattended. Make sure you have a fire extinguisher, good ventilation and a respirator. If you are at all uncertain about any of this, then don't do it, or do it with an older, wiser friend :)
2Build Oven Body
Lay eight CMUs on the floor together to make a pad. Make sure that the two CMUs in the center are on their side, so that their solid faces are pointing up.
Build up the walls of the oven with more CMUs. Use six CMUs for each layer of wall. If you are using Kanthal wire as the heating element, then you can lay just two layers. If you are using a propane burner as the heating element, then lay at least three layers of CMUs for the walls. Make sure that one CMU on the bottom wall layer is on it's side, so that the cores allow access to the interior of the oven. This is necessary for air flow and for running the gas line to the burner.
Lay bricks on the top layer of CMUs. These cover the open cores of the CMUs, and also can be moved around to accommodate the clamps and the lids.
3Installing the Heating Element
If you are using a propane burning as the heating element, then put the burner inside the oven. Run the gas line through the opening in the CMU. Lay a flat cast iron pan on top of the burner. Lay eight hollow core bricks on the flat cast iron pan.
If you are using Kanthal wire as the heating element, measure the resistance of the wire and calculate the length necessary to draw only 10A at 120V. You can run more current, but make sure that your circuit can handle it. If you make the Kanthal wire longer than necessary, you can always move the connections on the wire to lower the resistance.
Coil the length of Kanthal wire on a pipe. Cut short lengths of copper wire from Romex and remove insulation. These wires are used to suspend the heating element inside the oven. Stick the copper wire between the layers of CMUs to hold them in place, and make sure they are spaced out around the interior. Connect the coiled Kanthal wire to the copper wires with nuts and bolts.
Install the 20A light switch in the junction box and connect to a power cord and length of Romex. Run the Romex to the oven. Remove the outer insulation from the Romex to allow the hot and neutral to fit between different cracks in the CMUs. Connect hot to one end of the heating element, and connect neutral to the other end of the heating element, with nuts and bolts.
4Make Oven Lid
Cut MDF to two feet by two feet. Drill a hole in the center for the air fitting. Plug the hole and pour a silicone pad. Place a 12" x 12" piece of acrylic sheet over the silicone while it is curing, to make a smooth surface. If you are making a bigger oven, and will only be blowing one size of plastic, then you can save silicone by making a silicone seal around the edges, instead of pouring a pad. Cut slots for clamps to fit into lid. Install air fitting in hole.
Cut aperture for bubble. This will dictate the shape of the bubble. You can use a hand router, CNC router, or keyhole saw, etc. Finish the interior edges of the aperture, so that it is smooth. Find square metal rod to be used by the clamps to hold the aperture down.
Run the heating element with lid on oven. Do not load any plastic or connect air fittings yet. Use an oven thermometer or surface thermometer to measure temperature of oven. Once the oven is around 320F, take off lid and load plastic. Put plastic sheet on silicone pad. Put aperture over sheet. Clamp metal rods down. If using propane, turn off burner. Put Lid back on oven.
Rotate the lid periodically. Measure temperature of plastic. When plastic is around 270-300F, attach air fitting and turn on air compressor. Turn off electric heating element. Blow bubble until it is around six inches or more. Maintain air pressure and let bubble sit and cool down. Add additional air if bubble contracts. For better quality bubble, let anneal overnight. Remove bubble and unload from lid.