I would attach the box to your shoulders as they would be much more stable and would give you the freedom to move your head around without shaking your coinbox and possibly causing problems. Im sure some household items could be used to create a rig to hold up your box. I would say some sort of rod/pole that connects between your shoulder blades in some fashion and extends up and attaches to the box. I would say (depending where you will be) a clear rod or even one that is colored black so as not to draw attention to it. It would have to be thick enough to be shaken around and not have the fear of breaking on you, but not so thick as to be uncomfortable. Then just simple straps to hold it to your shoulders/back and viola, you have a coin above your head.
Btw, feel free to post some pictures of your finished costume. Im am curious to know what the final product will be.
Hope that helped some
