Hello John,
Several years ago I had a Folbot Greenland 2 skin shrink after extended storage (dissassembled).
I managed to stretch the skin out sufficiently to be able to pop the keel boards into place.
I laid the partly assembled hull out on a soft wooden surface, our deck, and I filled the hull with water. I let it sit for several hours, then emptied it. While still thoroughly wet, I slowly increased pressure while descending the keel boards and eventually, managed to coax the boards into place. I refilled the assembled boat with water again to "stretch" it to try to compensate for the shrinkage.
I kept the boat assembled after that, since it wasn't getting much use, and I didn't want to have to repeat the exercise with little assurance that it would work every time. Apparently, this was a fairly common problem with Folbot models when stored for long periods.
I learned about that issue and others from the wonderful Folding Kayak site that is now defunct.
Hope this helps.