The plastic part that holds the wagon handle to the wagon broke on our MacSports wagon bought through Costco. Didn't want to throw it out so I made a replacement piece. It reuses the riveted pins (that require removing from the old piece) and a couple bolts/nuts to hold the pieces together. Printed with PC-blend.

Hardware:
-original pin - used a metal cutting wheel to cleanly get this out
-original locking pin - used a metal cutting wheel to cleanly get this out
-2x 1/4 in.-20 Zinc Plated Hex Nut
-2x 1/4 in.-20 x 1 in. Zinc Plated Carriage Bolt
-Epoxy to secure the pins

Installation:
-Screw together over the rails ensuring the beveled edge faces the locking mechanism
-Press pins in and secure with some epoxy or other adhesive