If you decide to not put a lip on your coaming at the time you buid the boat you can add one easily later on. A simple approach is to use a piece of nylon or polyester rope saturated with epoxy resin. Wrap it around the coaming and it will harden into a lip. If you want it larger and more secure, cover it with a layer of glass cloth or 2 inch wide fiberglass tape -- using a few coats of resin to fill the weave.

David Hazen's book on stripbuilt canoes and kayaks shows a technique that might interest you, too. He cuts a 1/8th inch wide slot in the underside of the gunwales on his canoes. A fabric top wraps over that slit, and is held in place by a thin rope which is forced into the slot, trapping a fold of the fabric under it.

I don't need to post a picture of this. Go look at how your home's window screens are held into the aluminum channels. The screen mesh is trapped in a channel by a rubber or plastic "spline" strip.

A groove on the outside edges of your coaming would be a simple thing to do on a table saw if you construct the whole coaming in one piece, and cout cut the groove before mounting the coaming. Otherwise, a Dremel tool with a routerbase, or a router, using a 1/8th inch bit could do the job.

after you lay the fabric for your sprayskirt over the groove, a spline insertion tool (looks like a really dull pizza cutter) can be used to roll a piece of nylon cord into the grove to hold tha fabric in place. leave a couple of feet of excess cord at the ends, and knot the ends together, leaving a large loop.

To exit in a hurry, pull on the loop and the cord will come out of the groove, freeing the fabric, and the paddler. A velcro tab on the spray skirt will keep that loop close to you so you can find it easily, and yet keep it out of your way while paddling.

With this method of mounting sprayskirts you only need to fit the skirt to your body, and be sure you have enough fabric to overha\ng the coaming. If you have a bit too much, or evn 5 or 6 inches too much, it is no big deal. There is just a little extra overhang.