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Position the two 20" x 4" x 1" pieces of hardwood so that the base of the router will fit snugly between them. Measure the distance between the outside edges of the two pieces of hardwood.
Use 1-1/4" drywall screws to attach those two boards perpendicular across either end of the longer two boards (Image 1). Use a framing square to make sure the boards are perfectly perpendicular (Image 2). The four pieces will create a hollow rectangle.
Mount the rectangle to a worktable with the two shorter strips of wood facing down. Using a table saw, cut the 3/16" thick piece of clear plastic or Plexiglas the width of the router base and the length of the rectangle's inside length. Place the plastic in the center of the rectangle.
With a V-bit in the router, scribe a center line through the length of the plastic. The line will act as a guide. You'll place the work piece underneath the plastic and line up the spot on the work piece to be cut with the center line of the plastic.