Crawl space vent repair

Have a crawl space with what seems to be typical vents: a plastic box w/integral screening that is set into a notch at the top of the foundation when it was poured.
The screen has been damaged and has a hole in it. Hole is currently plugged with steel wool, but would like to make a more permanent repair. There doesn't appear to be a way to remove the plastic insert and replace it; can anyone suggest other techniques? Best idea I've had is to make a wooden framework with screening stapled to it and use tapcons to attach to the inside of the foundation.
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