Many thanks to all who have replied.
I already use 2 drybags clipped to the seat which have foul weather gear etc in.
I have spoken to a Tube repairer, and he believes that the tube material is not suitable for purpose.
The Bow stowage is very spectacular but impeades access to the bow, hence the reason for the attaching to the sponsons at the stern. (missed out that bit on original post, sorry!!).
I happen to know the RNLI 'E' class Lifeboats on the Thames have bags on their sponsons. It's something like that I'm after.
Its not the angle of the dangle, it's the motion of the ocean.