: Has anyone here ever taken a truck or tractor inner tube
: and put a stretcher frame in it to shape a Kayak-type floating
: device?

Yep, weighed a ton and the force to changed it to any shape except what it wanted to be defeated all attempts once it was inflated. Eventually I made a harness around it from soft nylon cord and the kids had a ball with it in the pool for years. These innertubes appear to be indestructible, it was passed on to another family - we were at least #2 in the sequence - and has since moved on elsewhere, still giving pleasure to children and frustration to any adult who tries to make it into what it isn't.