Fabricate the Hopper
Cut the sheet metal into a piece that will form a hopper 12" tall by 13" wide. The top of the hopper should form a square opening and taper from 13" on each side down to 4 1/2" by 8 1/2" at the bottom. Leave three inches extra at the top end of the piece to form a guard, and a 3/4" lip at the bottom for mounting. Leave an extra 3/4" strip of metal on the end of the piece of metal to provide enough room to bolt the hopper in shape.
Mark the sheet metal with lines indicating where the bends should be to form the hopper. Use a sharpie marker so that the marks will not rub off while you are working with the metal.
Place the sheet metal in a vise and bend into shape along the lines. You should bend the 3" guard and mounting flange last. Form the mounting flange by bending the bottom 3/4" of each side of the hopper 90 degrees toward the outside of the hopper. The end of the sheet metal forming the sides of the hopper should provide a tab that will overlap when you place the hopper in shape.
Drill three holes along the tab with the 5/16" drill bit. Place a hole at the top and bottom and one in the middle of the tab.
Form the hopper into its shape and mark each of the holes in the tab onto the face of the closing side of the hopper. This will show where to drill mating holes for the bolts to hold the hopper in shape.
Drill 5/16" holes in the places indicated by the marks made.
Place a 1/4-28 bolt through each of the three holes in the tab and affix a split washer and nut to each of the inside threads of the bolts. Tighten with a 7/16" wrench.
Cut the Feed Hole in the Mower Decking
Mark a hole on the lawnmower decking to bolt down the hopper. The hole should be 4 1/2" by 8 1/2". Ensure that the hole will be in the path of the rotating blade when the grinder is operating.
Drill starter holes at each corner of the marked opening. This will allow you a place to start sawing.
Remove the blade from the hack saw and feed the blade through one of the drilled holes. Reattach the blade to the saw handle.
Saw along each side of the marked hole until you can remove the metal plate. This provides an opening for the hopper to feed material to the grinder blades.
Mounting the Hopper
Mark the hopper with two holes, one at each end of the mounting flange on all sides. Drill a 5/16" hole at each mark.
Place the hopper over the hole in the mower deck and use the mounting holes as guides to mark the decking for mating holes. Remove the hopper.
Drill 5/16" holes in each of the marked locations.
Replace the hopper and place a 1/4-28 bolt through each hole. Secure the hopper by tightening a lock washer and nut on the underside of the decking for each bolt. Tighten each bolt using a 7/16" wrench.
Attaching the Supports for the Hopper
Use the vise to flatten each end of the two pieces of metal electrical tubing. Bend the flattened portion of the tubing in a 90-degree angle pointed upward.
Drill a 5/16" hole in each end. This will allow you to mount the supports to the decking and the hopper.
Place the supports against the hopper and the decking, one on each side of the hopper, and mark for hole placement.
Drill 5/16" holes in the hopper and the decking.
Attach the support to each side of the hopper and to the decking of the mower for stability. Use 1/4-28 bolts in each hole with a lock washer and nut on the inside of the hopper. Tighten the bolts securely.