since we're tossing ideas about... Aside from the installation hassle of the middle points, could you pop-rivet a washer (or more likely a disc with a properly sized hole in it) as a back-stop instead of just riveting to the beam itself?

From my experience, it wasn't the rivet that fails, but rather the mounting, requiring a slightly larger rivet to replace the failed one...?

How much weight did those spanners add to the beam?