Hi boothbay,
I believe I have a similar vent where I work.There was screening installed on the interior using pop rivits into the frame.You may need to build out the frame inside and you could probably fasten the screen using sheet metal screws if you don't want to rivit it.

Dear Boothbay,
I believe you are trying to close the barn door to stop the horse from getting out of the stall inside. Find the access holes for the tubing/pipe/wire and fill or otherwise seal off the entry points. Good Stuff, (expanding foam) works well and the little buggers won't try to chew through it.
Len

In think Iceman means "Great Stuff"

Got your e-mail, sent one back. First Mice are kinda like my in-laws. If you give them an entrance, food and shelter, they WILL keep coming back. Traps in the basement work well. Sheet metal below the hole in the floor works well also. I used Good/Great Stuff before and they didn't come back. They can't go through steel wool thats packed tightly. It will kill them internally. But as you've experienced, they are determined little buggers. I've even tried stuffing moth balls in the holes and they ate right through it. I guess they thought they were mouse breath mints. I finally won my battle. I had so many traps in my crawl space it looked like a Tom & Jerry cartoon. In desparation, I camped out in my crwalspace and shot 25 of them with a good pellet rifle. This is their time of year to further their species. And boy are they ever determined. Keep trying. The war continues.
Maybe I'll try the pellet rifle on my in-laws.
Len