I have a 1956 house near Los Angeles.

We replaced the whole HVAC system 6 years ago, including ductwork. The contractor removed all the old ducts, but we still have all the old wall registers. They covered up the openings behind the registers with what looks like Foil-Grip rolled mastic sealant (silver tape).

I want to figure out a more permanent solution and I’m starting with the old air intake (26”x17” opening) since I have other repairs to do on that wall. I peeled back the tape and it looks like it is just open to the raised foundation below, so I’m guessing it’s better to do more than just patch the interior wall. Insulation?

Our interior walls are rock lathe plaster - about 1/2” of plaster applied on top of something similar to drywall, if that makes a difference.

Anyway, looking for suggestions on something I can DIY, or what I should ask a contractor to do. The HVAC guy who installed our new system has since retired.