Well, if you're willing to install a small breaker box, then install a small subpanel - that's what those are. You need a 4-space main-lug subpanel.
The feed currently to the water heater goes to the main lugs.
A 30A breaker goes to the water heater.
A 20A breaker goes to the #12 going to the kiln.
This will trip the main panel if water heater cycles on while kiln is operating. If you want to interlock it so both loads can't be on at once, get a Siemens panel and this interlock intended for generators.
Big problem though. This panel won't have any neutral wire and it's a matter of time before some clown crams a 120V circuit in there. This will "apparently work" but be dangerous if you're using the built-in neutral bar as the ground bar. So I recommend you don't -- keep it as a neutral bar, just make it an empty one. Remove the ground screw from the neutral bar since it's a subpanel, then install an accessory ground bar and connect the grounds there. Leave the neutral bar disconnected. Bonus points if you find a panel that comes with an accesory ground bar, otherwise, they're like $6.
Breakers and ground bars must match the panel. For all practical purposes that means the same brand and series called for on the panel label, unless you are a genius on applying "classified" breakers, which are those specifically designed for competitor panels and are UL-listed to prove it.