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Modern white water kayaks aren’t all 4m long (as they used to be). Most are less than 3 and unless you are a big person, like shoes, you use the size which fits you. So lots are 2.5m ish.
This suited a not so big pal of mine, so he designed this set up, so he didn’t have to bother with ladders and roof racks.
Like a lot of van mods there are often compromises, in this case not being able to open the back doors.
The next design was offset from the towbar, towards the kerb, which allowed the right door to open, but lost the rear view for driving.
With access to a bit more fabrication gear, I think a mk 3 version could be mounted diagonally, from bottom left to top right. Which would retain rear view driving and accommodate a longer boat.
Thought this might help solve a problem for someone. Good luck