It'll prove best in the long run I think. The stringers could be pine,preferably spruce, or better still, beech. Although they won't be taking that much stress, damp, condensation and humidty/temperature fluctuations, over time, could cause some warping, and, with ply for stringers, possibly de-lamination.
Give the stringers a couple of coats of sanding sealer, followed by a coat or two of yacht varnish. The plywood former frames should be fine with just yacht varnish.
Have you decided yet on what to use for the 'fabric' covered areas?