The stress and strain on the upper edge of the plywood of your hull is he greatest on your boat. You don't want to leave this as an unsupported piece of 4mm plywood.

On a stitch and glue canoe you have TWO significant structural elements: an inwale and an outwale along this edge, or at least a strong gunwale on one side. This is further reinforced by at least ne thwart in a 10 foot canoe, 2 thwarts in a 13 foot canoe and 3 or more thwarts in a 16 foot or longer canoe. sometimes seates are mounted and serve as bracing inplace of thwarts.

You MUST have this bracing or the water pressure on the outside of the hull will cause the shape to change. In an extreme case it will cause your boat to collapse in on you when the boat is loaded. The effect is something like being held in the jaws of a man-eating clam. Depending on how it happens, your chances of escape range from either very simple, to nearly impossible.

With a kayak with a wooden deck you have a piece of plywood, or a strip structure, usually at nearly right angles to the hull's side, and that serves the same purpose as the thwarts in a canoe. You have attached ribs for supporting the canvas deck, and they should help, but they really need to be pushing against a sheer chine, or gunwale strip in order to distribute the loads along the length of the hull. If you like the lines of the boat as you have it, glue or screw an inwale strip inside the hull as close to the top of your plywood edge as you can get it. If need be, make a few notches in these strips to fit them around the ends of your deck supports.

Use a soft wood like pine for this. Make this at least 3/8th inch thick (1/2 inch or 5/8 inch would be better) and 3/4 inch to 1 1/2 inches wide. Once the deck is installed this strip will be unseen. This strip will serve a further advantage. You can staple or tack your deck fabric to the top inch or so of the side of the hull, using staples or tacks that will go through the 4 mm plywood and into the pine strip behind it.

Hide the rough edge of the canvas with an outer rub strip that is at least 3/8th inch thick, and which is screwed firmly into the inner strip. Glue is not wanted here. Eventually your rub rail or canvas top will get dinged up enough that you'll want to replace it.

The screws work to lock the inner chine or inwale to the plywood hull and the outwale or rub strip, giving a solid support to the top edge of the hull. You then unscrew things for the repairs. Your deck canvas supports then act as spreaders or ribs or thwarts (call them what you will) and keep the boat's shape.

Remember, when you a decided NOT to build the deck according to the plans you changed our boat from a kayak to a canvas-decked canoe. Slightly different rules apply. When you take off the kayak parts ya gotta put on the canoe parts.

Hope this helps

PGJ