Stuff the areas with steel wool.

Start putting out a small bowl of cat food everyday. A feral cat will show up. If you're lucky, it will also be a good hunter. We had one for quite a few years that totally eliminated any rodent problems in both our home area (garage) and barn (horses). ; )

Don't seal it tight; it may need to drain water and should be able to dry out.

No steel wool; it rusts quickly.

Just stick some balled up window screening in there (not a tight ball that would impede air and water). Or cut up one of those stainless steel coiled scrubbers.

RV people like to mouse proof with brass wool--doesn't rust like the steel wool.

We have 2 indoor-outdoor cats who catch about 4 rodents per week, each. They can't seem to keep up. There's woods across the street and behind the house so there's lots of habitat for wild critters.

Never heard of brass wool, I'll have to ask at both local hardware stores.

I've used both steel wool and wadded-up aluminum screen wire, followed up with a nice big glob of silicone seal. Both seemed to do the job. But I can see how steel wool might rust if there's too much moisture around.

Bruce's idea sounds good too. Lots of sharp points to poke those little trespassers in the nose. Yeah!

Cats are worth having, too. Most of my barn rat problems vanished after I got a pair of barn cats. It took a few months, but I guess that the mama rats eventually decided the neighborhood wasn't so great for raising kids and moved out.

Rats are smart, but mice seem incredibly dumb. It's like they play chicken with each other. I've seen them run right out in front of a cat (and what usually follows, at least with relatively young and active cats). I've always heard that cats play with mice until they (the mice) break, but (warning: graphic description ahead) mine generally just crunched 'em and swallowed 'em whole. Snacks beat toys, I guess.

Our cats show-off their catch, usually in the hallway. If we happen to still be in bed (door closed), then I get to dodge a gizzard in the hallway the next morning. Once in a while a bird will get away from them, which I then have to catch and release.

The man at the construction desk at Lowes suggested a silicone caulk and steel wool combo. He said to put the silicone inside the open channel then press a big was of steel wool into it.

Hard to find local yearly totals, but the major reporting areas near me had over 50" of rain in 2018. Our usually dry 8-10 weeks in late summer became one week with no rain. The farther up the channel I can place the critter block, the longer it'll take to rust out.

Sounds like it is the best choice, even though it will look like a chewing gum & baling wire approach.

Thank you for your helpful ideas and experience.

Get either the brass wool or stainless steel wool or, probably cheaper, some of those stainless steel pot scrubbers. Not plain steel wool; I swear that stuff will rust under your very eyes.

Pot scrubbers might be the easiest to find. I'll look for those on my Monday shopping trip.

"Our cats show-off their catch...."

Mine will do that if I'm not around when they bag the varmint. The barn cats would sometimes leave nearly whole mice and chipmunks right in front of the door for me. Once in a while I'd find a squirrel, but just the tail (I guess squirrels are more tasty).

Once I went away for a weekend, leaving plenty of water and timed-release food for my cat. When I got back Sunday night and went into the bedroom, I noticed an "uh-oh, something died" aroma. She'd parked a mouse right in the mathematical center of the bed. You know, just in case I was hungry when I got home.

Cats aren't for the squeamish, I guess, but as long as they're young and active, they're pretty darned effective against little furry nuisances, and no poisons are needed. I'll admit that they're not the most humane way to dispatch mice, but then neither are glue traps.

They do seem to lose their edge with age, though. I've read that most cats' visual acuity declines quite a bit over the years, so that might explain it. I haven't had much luck finding an optometrist willing to fit my older ones with specs. Maybe contacts? :)