See if you've got feed at a glance. This clear polycarbonate sight glass installs in virtually any feeder. Simply cut out a 6" x 1.5" slot in your hopper, run a bead of silicone around the flange, and screw (or bolt, or rivet) into place. Made to install on the inside of the hopper.
Overall dimensions: 7" x 3"
Window dimensions: 6" x 1.25"