The lip can be made of multiple pieces, which keeps the size of the board (and steam box) you need down. Run a tape measure around the vertical coaming rim and add the length of the scarf(s) you'll be using. If you use 1/4" strips, each successive layer will increase in length by ~1.5" (Pi x the increase in the diameter). There are pictures of coaming construction using this method in my SOF Jigs and Tools album and on Webshots and pics of finished coamings in the other SOF albums. The lip is made in the same manner as the rim, using the rim as a form. I use 2, 1/4" x 1/4" laminations for the rim, but you can use three if you want a deeper rim. I recommend rounding off the edge of your stock prior to cutting the laminations for the coaming (I do it on a router table). It's much easier than trying to round it off after it's bent.

Why do want to make the lip 1/2" high? A thickness of 1/4" is plenty and helps to keep the overall height of the coaming down.