Re: what did you do to your motorized bicycle today?
I once mounted that style tank by cutting two 2'' rings from a pipe of the correct size and welding them to a crossbar that in turn welded to the frame. Two rubber caps slipped onto the rings made it snug and secure just like a factory mount. The rear used a u-bolt like you describe.
I think this would work quite well for a bicycle mount. Sorry no pics as it was back in the early 80's and camera phones didn't exist yet.
If more is better, then too much is just enough.
If you can't afford it, build it yourself.