I have got a better idea, provided you are somewhat familiar with a tad more than very basic carpentry.
Cut the length of the panels to 3 inches less than what they are now. Get a good hardwood, say oak, and plain it down to 1 1/2 inches by the exact width of the plywood. They will go on the two ends of the panels. Attach them with biscuits, and Titebond III glue (completely waterproof). If you don't have access to a biscuit jointer, 3 inch galvanized finishing nailgun nails should do the trick. Just be sure to clamp it well with pipe clamps, and completely cover the edge of plywood with the glue, as it will be about the best water sealer you can come up with. Now, put your panels in place, and screw through the ribs you do have, into the new piece of oak with stainless screws (you will need pilot holes into a hardwood).
It may seem like a whole bunch of work, but in a year or so, the screws into the endgrain of the plywood will work loose. The hardwood will prevent that for as long as the rest of the wood lasts.