Firewater,
You do not want to control the temperature; the boiler will get as hot as the boiling liquid inside. If you are boiling water it will be 100 deg C if you are boiling a wash it will be lower, the more alcohol the lower the boiling point.
The more power you apply the more violent the boil and the more vapour will be produced but the temperature will not increase. When you have your power controller installed you will be able to dial in maximum power to give maximum vapour for a stripping run and reduced power for a gentle rolling boil for a spirit run.
The washing machine door seal sounds ingenious, but do you know what it is made of

There are many forms of rubber and plastic but most are not safe to use, many forms of rubber / plastic will leach harmful chemicals into the still. Silicone sealer is a good example, there are many varieties available and most have additives for specific purposes, ie to reduce mould many companies introduce chemicals that are poisonous to the mould and unfortunately humans as well.
Unfortunately I cannot recall the name of the silicone sealer that I used; I did buy it from a dealer selling Koi Carp and all the paraphernalia that goes along with them. These dealers sell a sealer without additives for making aquariums. Silicone appears to be inert and suitable for our purposes once cured, but releases chemicals during the curing period.
Please be sure that whatever seal you use is inert and fully resistant to alcohol and is fully cured. If in the slightest doubt about the suitability of a sealer then either use flour and water paste, or alternatively, wrap a cardboard gasket with a couple of layers of PTFE tape.