Check the size of your 1/2-in. plywood backing board. Then, cut the fiberglass window screen to be eight inches larger than it in both directions (so there are four inches of overhang on each side).

Does it make sense to place 5 GHz capable WAPs behind plaster board (I guess that's what they are called: gypsum backed with paper)? I'm thinking about where to place some Unifi WAPs. How much signal ...