This is for a rental unit, so I'm trying to keep the cost low, while also sealing it away from silverfish. I have very fine steel wool on hand as well as a tube of DAP ultra clear flexible all purpose sealant.
I work in the power industry, we use foam to fill ducts. The inside of the duct is filled with little rubber tubes (I think 1" diameter, maybe smaller), these provide structure for the foam to fill bigger holes. Without that though it isn't suitable.
Spray foam the void if possible, but most spray foams will only expand up to 2 inches. This provides a bit of insulation and protection against pests.
Might be the angle, but those shut-offs look a bit close to the wall, might be hard to do anything that will finish it off/cover the spray foam.
I work in the power industry, we use foam to fill ducts. The inside of the duct is filled with little rubber tubes (I think 1" diameter, maybe smaller), these provide structure for the foam to fill bigger holes. Without that though it isn't suitable.