The slant backs I have owned have snaps all of the way around the cockpit, outside the gunnel. You can put a vinyl awning rail, and then slide the forward part of the slant back down into the rail, which gives a watertight connection forward along the side of the cabin house. With this set up there will be little water getting, if any. But you also loose vision aft (Back up camera from Amazon "solves" this issue, or as I note below, build a window in the slant back.
It is easy to make a slant back out of shelter rite, which is a vinyl material with Dacron scrim--similar to the material that inflatable boats are made of. Seams are glued with HH-66, and destructive testing shows fabric fails before the glue line. Snaps are easy to set along the edge-I would double or triple the fabric where the snap is placed. Awning rail rope by Keder glues in nicely, since it uses a similar vinyl. (See Sail Rite website). I made a full camper back out of this material with full windows all around--if you wanted to glue in a clear vinyl window in the slant back, it would be easy to do.