Neill,
I really don't think this OS Sander Plate is for you. It would be a mistake to purchase this plate and attempt to cut it down to fit your router table. There are leveling screws around the perimeter of the plate, so cutting an inch off of one end would then also cut off the two leveling screws on that end. Worse yet, the end you would likely want to cut off would be the same end that has a spring-loaded gizmo installed into the edge that keeps the plate snug within your table's opening. No, I would not recommend this product for those with a router table top that requires a different size plate. If you want to modify a plate to accept your PC-121, you would be best to start with a blank plate that is made for your table.