We needed to pet-proof the screened door. I didn’t like the commercially available RV pet grates because they had poor reviews and appeared flimsy.
I settled on aluminum flattened expanded mesh. None of the local hardware stores had it so I ordered a piece from onlinemetals.com It was expensive, but they cut to size, so I didn’t have to mess with getting straight edges. The size needed for a Casita door is 20″x18.5″. The panel was $25 plus $20 for shipping. You might be able to find cheaper mesh at your local hardware store.
I painted both sides with Rusoleum Satin Black spray paint. To install the guard, I used #8 x 1/2″ long stainless sheet metal screws and washers after drilling 1/8″ holes in the door frame. It came out great and no more worries about the dog turning the screen into a “doggie door”.