Upper hopper features a warped 1/8" ply false floor so balls roll to the center. I added the cardboard semi-circles to direct the balls into the hole (glued with Elmer's Carpenters Glue). If my 9th grade geometry volume calculations are close, upper hopper should hold over 2000 balls.

PVC down tube has a plywood slide gate to keep balls from draining from hopper when machine is opened. Gate is made from 1/8" plywood. Holes are cut to fit the PVC (1.5" in this case). PVC is glued into top ply with Household Goop glue.

Completed slide gate assembly.  Dowel rod is attached to slide with a paperclip slid through holes drilled in dowel and slide. The safety pin is attached through a hole drilled in the dowel and stops against the cabinet to keep rod from being extended too far.

Slide gate installed and in action. The PVC is wrapped with masking tape and pressure fitted to the hole in the cabinet so that the bottom of the assembly is within a few millimeters of the machine's hopper (far less then a ball's diameter). I tested the slide gate with the machine's hopper and PVC down tube full of balls. Not one ball fell when the machine was opened. The gate works perfectly (YMMV).

View of French Cleat mount (for more info Google “French Cleat”). Cleat is attached to wall studs using (4) 3" lag screws and washers. Drill pilot holes in the studs so they don't split!  The French Cleat makes the cabinet a breeze to install.