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You can do some measurements.

From the centerline of the hull, measure out 2' horizontally. Now measure up to the hull.

That is the distance, plus ~3-4", the bunk riser needs to be. They make them up to 12" tall. If you need more height you can add blocks under the bunk boards if need be.

Also measure the height of the fender off the frame. This distance is also important.