Product review (5 stars): Crawl Space Vents
Very sturdy plastic vent.  I used them for crawl space vents - much more attractive than plain ole galvanized screened vents.

I installed perforated aluminum sheet "screens" into the back side of the vent before installing the vent.  By cutting the perf aluminum with sheet metal snips very slightly larger than inside dimensions of vent, the "screen" is wedged into the vent tightly, avoiding fasteners and labor time.  (The rectangular sleeve at the back side of the vent is thick and sturdy plastic construction).

I used aluminum perforated plate obtained at Online Metals, 1/4" dia holes, with free area equal to the free area of these vents - just measure the open area of the four slots in the vent and select a perforated plate that matches this free area.  Uniform Residential Code requires maximum screen opening size of 1/4-inch.