This is what we did using scrap wood and an old sheet of scratched plexiglass screwed to a frame made from strapping. Added another frame with hardware cloth on the inside:
The pop door you see is double-latched and has two pieces, once on the outside and then on the inside so that if anything gets through one door, it faces another barrier.


The pop door you see is double-latched and has two pieces, once on the outside and then on the inside so that if anything gets through one door, it faces another barrier.