Even if you make them yourself, slatted wooden bases for beds and settees are much to be preferred over plywood bases. Mine are made from 70mm x 20mm softwood, varnished, spaced 50mm apart, and the mattress and cushions can breathe well. In contrast, plywood bases, even when perforated by large holes, tend to encourage black mildew on the underside of the cushions, when in full-time use.