[dsraven]

actually you could use a muffler clamp to hold the vertical fill tube as long as you don't mind the fasteners protruding into the bed. you could cut the u bolt part off at the right place to allow a long nut, like they us for threaded rod, to be installed on the clamp instead of the regular nutys that come with it and then the whole thing could be mounted to the box side with bolts from the inside of the box into the other end of the long nut. cut the nut off a bit so the affair isn't spaced off the box too far.
just an idea.