One idea I had was to try to find something like a pipe repair clamp
(the thing that clamps onto a water pipe to repair a leak), but really
heavy. I could then clamp this onto the axle of the wheel when needed
and unclamp it when I don't. There's a handle on the front side of the
can that I could also clamp something to in order to balance the extra
weight. Unfortunately, I haven't been able to find anything that would
do the job.