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The knee wedge serve a very important function of locking your legs in place and keeping your knee caps from rotating as you look over your shoulder. The wedge also serves the important function of serving as the center wall.
Start with a short piece as if you are making a short section of front wall. If the piece you have is not big enough you can glue a piece on top as shown in the picture above. Shave for fine adjustment if necessary. You want this piece to fit snugly to eliminate flex. You slide this piece in after you get in the boat. Typically it sticks out past the edge of the rim a few inches.
Glue small pieces on the top sides of this piece. These pieces need to be there but they don't need to be very big. Glue them in place first and then cut smaller and shave to fit. Remember that if you have your blue jeans on while you are outfitting that things will be looser in your paddle clothes. The wedge should be tight enough to take a little force to get in.
A piece of rope is necessary to ensure that the block can be pulled out in a hurry. You can make a pilot hole for this with a screwdriver. If you insert a piece of PVC pipe first it will keep the rope from pulling through the block as fast.