I've chopped up something like 2.5metres of 2020 aluminium extrusion and turned it into a more permanent, more rigid frame for the drum feeder, vibration feeder and sensor head:

The drum is belt driven from a DC motor/reducer gearbox running at about 200rpm. I needn't have worried about the belt wandering off the drum - it's absolutely fine, without any need for additional guides or track to keep it centred. 

Everything's adjustable - angles, clearances, etc. - so the next steps are to do some tuning to get it feeding reliably and quickly, before finally integrating it with the diverter and output bins.