Hi Clemens,
The rod that forms the coaming is currently a single piece of 3/8″ OD, 1/4″ ID Polyethylene (PEX) tubing. This is available at most hardware stores in North America for installation in household plumbing as an alternative to copper or CPVC.
There is a short section of 1/4″ aluminum tube inserted as a joining piece.
I believe this is what Alpacka and others are doing on the current crop of whitewater oriented packrafts.
Following Matt’s construction guide for a spray deck, I have a small slit in the inside rear of the cockpit that I feed the coaming into and snap the joint together through.
I will be making some improvements to this area as it’s the one thing I’m not 100% happy with. It is currently very difficult to install an off the shelf whitewater kayak bungee sprayskirt. However, once it’s on, it’s fairly secure.
I think a slightly longer coaming piece than I currently have will create a deeper and more secure lip for the skirt to fit over and make it easier to snap into place.
Hope this helps.