Position a 24-inch board against the pallet's edges on each side so they're perpendicular to the boards on the pallet. Center them using your tap measure so the distance from the ends of these boards to the corners of the pallet are equal on both sides. Screw through the inside of the pallet and into the 24-inch boards, using eight screws for each board.
Drill two 1-inch holes that are 3 inches deep into each 24-inch board so they are 1 1/2 inches away from the ends of the board. The holes are perpendicular to the pallet.
Screw one end of each 30-inch pipe into an elbow. Screw the 20-inch pipes into a pair of elbows so you have an upside-down U created by the pipes.
Screw a pair of swivel wheels to the bottom of each 24-inch board so they're flush with the ends. Insert the pipes into the 1-inch holes.