Get some muffler clamps, run them around the bar, and then use back-plates inside the box to secure them. Then just mount your speakers to the front of the box, and screw it all together. If your going to make the boxes yourself, use some 3/4" particle-board or MDF. You could then carpet it yourself with what-ever you choose (don't recommend shag though).
You could also make them "removable", and attach some extra speaker cord so that you could pull out the speakers and place them where you want. Did this several years ago, and it was really cool at parties to be able to hang the speakers where I wanted them.
There are several places that make "hangers" for heavy objects like flat screen TV's etc. that you could modify to hang the speaker boxes from.
Erik
