Repair/replace foundation vents in wooden siding?
The vents you removed were constructed entirely of 1x2 lumber and could be easily recreated. You'd want to use cedar or pressure-treated lumber for rot-resistance, if not a synthetic.
All cuts are square. Some corrosion-resistant screws, properly piloted and countersunk, would make them solid. Trim-head screws or splitless siding nails would secure them well.
Use a good quality exterior caulk to seal around the outside, then prime and paint.