Heiko,

I've been thinking this problem over as well. The one bicycle-towed kayak trailer I've seen was built with two wheels and a single round tube that runs lengthwise between the carriage and a ball joint affixed to the seat post. There is a way to build one that is simpler and lighter and would require less storage space.

The kayak is rigid so any lengthwise tubing you'd use would be essentially redundant. The carriage could be built up starting with a padded yoke that straps snugly to the form of the kayak at, say, two thirds back from the bow. Add a pair of struts for stability and situate the thing so that the boat sits fairly low between the wheels. Make another yoke or arrangment for the front of the boat and attach the towbar there. In essence, you'd just be adding temporary wheels and towbar to the kayak. It would take up little space when not in use.

If you'd seriously consider something like this, I can whip up an AutoCAD drawing of my idea for review/modification. There must be more bicycling kayakers out there with some nifty ideas.