@SirNuke All of the comments saying spray foam are technically correct - that is the easiest way but it will look like orange crap. Some people mentioned steel wool - avoid that because it rusts and will end up looking like orange crap. Find some "excluder mesh", which an inexpensive product sold to the pest control industry - it is a nylon mesh with reinforcing fibers...think Brillo pad stiffness. Stuff the gap to fill it then use silicone sealant to finish it off.
@SirNuke The other reason to not use steel wool is that it can be a fire hazard. A low voltage shorted across steel wool will result in fire (I carry a 9-volt and steel wool in my camping kit as emergency fire-starter).
@SirNuke Another option if you don’t want to source excluder mesh is to get some “backer rod” - a painter’s product sold as “caulk saver” at the home improvement palace. You use it to fill large gaps which you then seal over with caulk or silicone.
I would say the expanding foam would still be preferred. Just don't go crazy with it. Then before it's fully cured, cut it back flush. Wait for it to dry then caulk to cover/seal.
My actual job is to inspect homes. It is an industry joke that Joe-diy-guy grabs a can of great-stuff expando foam and “fixes” it…leaving a jacked up mess that rats just chew through to get in again. There are better ways of securing entry points.
