Plywood will definitely sag very quickly, but another good reason to stay away from wood is mites. Mites and their eggs are very difficult to eradicate from wood, and wood is a haven for those little white wood mites as well. The white mites are not necessarily harmful, but they're just a pest you won't want to deal with. I'd stick with plastics and metal frames. It costs a bit more, but it's well worth a clean, healthy collection.