The Pygmy Arctic Tern (S$G) instructions seem to anticipate there will be trouble with the keel seam in the middle half of the kayak. They suggest you lay the boat's keel on a 5 foot long 2 x 4, while propping up other areas as needed to hold the kayak securely, and then stick large push pins from inside the cockpit area along the keel seam through the seam and into the 2 x 4. This assures that the keel seam remains straight and flat while the interior seam gets tacked and filleted. I think it's pretty important to keep that area fair!

Here's a tip: Remove the 2 x 4 prior to launching and the boat will be easier to turn!