My washer boards are 2' X 2' with a 2" X 4" raising the back up and a 4.5" hole in the center. The washers I use are a little bigger. I'll have to take some pictures and get the measurements so I can post.
Mark the area with a pencil using the PVC pipe as a stencil (inside and outside) This will let you screw in the right zone. I would recommend drilling small 1/8" pilot holes through the wood into the PVC where the screws will go first, then screw them in.