I was just looking at a skin yak frame sketch and I think it would be fairly simple to shape an ethafoam bulkhead before and place it befor the skin went on. You could dish it out a bit so it doesn't transfer through the skin and then seal with our buddy silicone after the skin is stretched. Since it would be sealed to the stringers it would also be fairly solid.

A variation is to do the above but cove the edge of the ethafoam. Place a bead of silicone all around the bulkhead and frame and press in a 1" diam bike tube. Let the air out and put on and stretch the skin. Put another bead of silicone betw the tube and the skin and blow up the tube . let set. let out the air and you have a pretty good flexible seal.

- dumb but may work? -mike