Affordable but durable mesh wire for crawl space vents?

If the wires are round, a simple pair of wire cutters will do the job.

Let's say the opening in the cinder block foundation is 18 in. wide by 10 in. high, cut the wire 22 in. by 14 in. so you have a 2 in. piece you can bend to form what looks like a box lid.

Bend the 2 inches all around just shy of 90 degrees then from the outside of the foundation reach under the house with the wire cover, spin it so it is properly oriented and with the bends facing inward then pull it back toward you.

This should allow the less than 90 degree bends grip into the cinder block by friction and should be strong enough to keep a cat out.