I have experience with string drive – I’ll see if I can get you a good picture of my ‘grommet drive’ mechanism – it’s a super cheap stepper drive system that gets a good grip on the braided fishing line I use.
Simply put, it’s a large grommet on a brass compression pipe fitting on a stepper shaft.
As for cleanup, I highly suggest a vacuum system – have the bottom section be a MDF sidewalls instead of aluminum rails, with a canvas sheet on the bottom with a port for attaching a shop vacuum hose.
Cleanup is then a matter of turning on the vacuum, and dusting extra powder off the top into the piston chamber, where it falls into the canvas and gets collected by the vacuum.
Or just falls by gravity into a collection bucket.