It's a tricky thing to incorporate the lock in the knob and spool (shaft), so consider another way. For example, you could put a threaded hole in the front/rear standard by the rail and have a nylon ball in the shaft backed up by a bolt with knob at the end (they sell the knobs w/bolts at ACE Hardware). A tiny bit of pressure on the shaft seats the nylon ball just enough. You should not need much pressure at all.