Off the wall maybe, but what about elastic bands?
Decent rubber band used for catapults etc could pass through the centre of tube from one sponson through the tub to the other, like a tent pole, and hold the whole thing in compression, you just need some form of cup to allow a knot in the elastic to site flush on the sides of the sponsons, and the shafts arranged so they socket together to a shoulder. It would be no problem to stretch the band out before knotting to get a firm setup.
I'll get my coat
