I've used multi-clamps like the
Pearl AX25L on square tubing before and they held pretty well. The openings in the clamp were a little bit squared-off and it wasn't hard to situate them to get a good grip.
Getting something attached to your marimba frame might be a little trickier, though. Maybe one of the clamps for
Pearl's square-tubed drum set racks (like
this one) can clamp to the frame?