Got together with Mr Angle Grinder & his friend Mighty Hammer! & fashioned a seat base today.
I had an old dual seat from my Royal Enfield sitting under the bench for several years & thought that it would be a good starting point...
Not a straight bolt on though!!!
You can't really see it in the pictures, but I managed to get a turn down all round that stiffened up the whole thing. I'm now waiting for the rear mudguard to turn up so that I can make the rear mounting, then I can get the foam padding from the donor seat shaped.