The drum feeder is looking good now. After a bit of trial and error making paddles/scoops from card I've committed to a 3D printed design that works well enough for a first attempt. Physically positioning all the various parts was getting really tricky using the wooden lash-up so have also started the 'finished' build in 2020 profile and M5 fittings.

I think I'll try a belt-drive to power it, from a DC motor and reducing gearbox. Only problem with a belt is that the drum has a slight draft to the sides, meaning that the belt will undoubtedly tend to wander off...