Reservoir/Overflow Inverter Tank Leak??

Post #15 used some Permatex-in-a-tube as an alternative to the real seal, seemed to be satisfied. There are so many different flavors of Permatex sealants it'd be hard to say what to recommend, to say nothing of chemical compatibility with SLLC, but the option's out there, anyway.

Another option would probably be your local hardware store, which probably has a display case with dozens of little drawers of differently-sized O rings. You could probably pick up a few that are the right inside diameter to fit over the tank spigot, with different ODs/thicknesses to see if one will let you snug the tank up to the inverter and make a good seal (without being so thick you risk tank damage when you snug down the bolts).

That would probably easily serve until your real seal arrives at the dealer (and then be easy to swap out, unlike trying to deal with cured Permatex...).

-Chap