You're right, a triac would most likely fail short.
I think you're making it too complicated, though. Why not start with a 2000w/240v element. It will have 1/4 power at 120v. Now it's 500w. Throw a diode in, and it's only getting power half the time, so now it's 250w. If that's not low enough, then put two in series instead, and you have 125w.
If you're talking lower power like this, I'm not sure I see much need to bother with immersed elements. I'd slap some PTC heater elements on the outside, possibly insulate it, and call it done.
Last edited by Stubby79; 01-02-2017 at 02:39 PM..