This a catch-all update for the progress I made before making this project post. I constructed a hopper using scrap wood I had around my garage. Most of it is cedar to prevent rot over time. the white section is PVC because I ran out of cedar... anyway there are two 8" pieces and two 6" pieces mitered to 45 degrees and joined with screws. It connects to the shredder with four 1/4 inch dowels. I drilled holes in the shredder frame to fit the dowels, and fused the plastic infill together with a soldering iron and filament. This was kind of a hack job. My joinery skills are less than stellar and I had to re-drill the holes for the dowels so they'd actually fit. Having said that, I'm not exactly going for style points here. I just want something that works and will last a while.
I also printed a custom part to connect the printed hex shaft on the shredder to the 25 tooth sprocket. After this thing is up and running I will do some testing to determine whether this design is good enough to handle the load it will be exposed to. Ideally I'd machine a direct connection into the sprocket itself, but I don't have reliable access to a milling machine.