I use good old duck tape. Remember when you mention screen, nothing metal can be used as it's toxic to inverts. The GPO at the Baltimore Aquarium had a type of mesh attached to 2x4's bolted with cement blocks on top. Granted you don't need to be this extreme but I have always used a glass canopy and then proceeded to duck tape all openings.
Have you had saltwater experience? Check out the great threads and info under Cephcare.
Yeah i have experiece, Iwork at a store in Charlotte NC just havent ever kept an octopus before so i want to take precautions. I hadent planned on using metal screan more of a window screen material but thanks for the heads up. I appreciate the reply.