Heater box reassembly: I still have questions.

[In The Ten Ring]

I need to get the heater box back together and back on the truck this week. Several members suggested I used "gum cord" or "strip caulk" but the HVAC store here does not know what those terms are.

Would I simply ask for a gasket or sealant in a tube at the auto parts store?

Who has reassembled their outer (under hood) non AC box? What were your exact steps to do it?

*Sorry if I come off as a moron, it's my natural state sometimes.