The only reason to have one thinner than the other is to identify one from the other quickly when inserting them. The drawings describe a pair of keepers that will be lighter than solid wood and just as strong. Note the bearings - these are critical to a good rudder keeper. One pair at the top and one pair at the bottom. Bearing buttons of Teflon, Delrin, Nylon, or almost any other plastic are screwed into the boat at the top and bottom of the forward end of the well.
Similar buttons go on the front of the forward keeper.
In making the keepers, take measurements of the well on your boat - don't build from a different boat's dimensions as they not all the same. Shim the upper and lover spacers to take out excess slop, but don't make a tight fit. It's good to have the bottom spacers fill most of the hole to avoid the drag of wide cracks or protruding corners.