The vents holes under my sister's house are one cinder block square.

The cheap vent covers were nailed in place by just driving nails into
the mortar joints. Her dogs push them out to get under her house.

My solution will be to just take some one inch thick wood and cut the
pieces the same size as the cinder block and wedge them around the
opening and screw the vents to the wood.

Any better solutions?