Many thanks for replies they have all helped me to formulate a plan.
I also had a bit of luck today, I (re)remembered that one of my daughters school friends has a father who owns a steel supply and fabrication shop
I went and had a chat with him today and he is going to be a great resource for materials and some fabrication help.
I am thinking of trying to center the support for the yak under the strongest areas (the drive wells I believe). To do this I would move it further back on the trailer and support the central section with 2 lengths of PVC pipe of 6 or 7 feet. Then provide additional support to the rear by running a single tongue from the front, under the bed and out about 4' behind. Possibly making this removeable to aid loading/unloading. Not an original design but a cheaper, steel version of this one....
viewtopic.php?f=73&t=45956 created by Lomcevak.
The only drawback that I can see is that maybe the reduced tongue weight may effect it's towing characteristics, but I could always use the sway control from my popup trailer.
I may lash the amas on top in the same way as I do when car topping. I have padded the cross rails with pool noodles cut to fit and then I reverse the amas with there sterns pointing forward and snug them down with cam locks. They fit very well this way around and I have travelled about a thousand miles with loaded this way this summer without issue. I was worried about how badly all that on the roof would effect fuel consumption but was surprised to find it only cost about another 2 mpg over just pulling the camper.
Thanks again,
Jeremy.
P.s. I was going to post a photo of my car-topping set up but don't seem to be able to add attachments?