i've built several sets of dark-side knees, i make them as adjustable as possible. i just cut off the fiberglass tab and the excess left behind, then i cut a one half to three quarter inch by two and a half inch strip of 1/8" aluminum bar. then you scuff it up real good and epoxy a little over half the aluminum sticking out; clamp it and wait for it to cure. then leave about a 1/8" gap between the two pieces when you epoxy it to the front. once that is all cured, put a fiberglass patch over each end of the aluminum bar. you're basically building a bridge between the two parts.
once everything has cured for at least 24 hours you can adjust the width of the knee by grasping firmly to each side, then gently apply even pressure to push or pull the sides for proper fit.