Re: Making a jacketed, electric bottling tank: how are the prefab ones set up?
Your double tank sounds complicated to me.
Instead of the outer tank, wrap the keg with flexible heat tape, then use expanding foam in a can to hold it in place and provide insulation to reduce the heat loss. You could use a plastic garbage can to provide a form and outer casing for the tank if you want it to look neat. After the foam has hardened, cut away the plastic and foam to provide access to the honey gate area. Of course, do the required modifications to the keg before foaming.
Disclaimer: I have not done this.
-- Victor Hugo -- "Common sense is in spite of, not the result of, education.”