Thanks for the good ideas. I was considering something similar, beginning with half round molding (like you can buy at Home Depot etc for stairway handrails, and then doing the cut and paste method. The built in flare is pretty close to what I would want to end up with and would save a lot of tricky shaping. I particularly like the idea of plastic bags on deck so you can construct coaming in its entirety and then decide whether it is good enough to be attached.

Thanks, Dave