Dixon Bate do a "build it yourself" thing (like a giant meccano set) which is relatively cheap and due to the "bolt together" nature means ou can alter the drawbar length.
Alternatively as it's a car top type dinghy, could you do what some of the multi laser trailers do, but a "solo" version - if it's only doing a couple of miles every so often - buy a cheap box trailer & fix a roof rack to it? I saw recently a "tinnobox" trailer at either B&Q or Halfords - one of them had it with the roof rack on as the demo trailer outside. Then you get a useful box traielr into the bargain!
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