for less than another $100 you could use black-pipe to hold the water heater element, and rubber heater hose to put the sources of ignition on the other side of a wall…or at least 20ft away from your solvent tank.
there are pictures around here somewhere.
if you use black pipe, you can then use a standard hot water heater thermostat as your over temp protection.
if you want to get more fancy, glacier tanks sells tri-clamp adapters that will take a hot water heater element, so you can make a nice shiny version for less than $300.