This sled is designed to ride between two cleats along the two parallel sides of your router table top. If your table top is irregular, you will need to temporarily clamp two parallel cleats to your table-top edges or rip one side parallel to the opposite one.
Measure the width of your router table top and add 2” for the two 1” cleats that will be installed later. Using 3/8” or 1/2” plywood or MDF, cut a piece to that measurement for the bed of your sled.