without reading all the previous replies....I use my extender but i dont worryabout sliding it.....if you do want to keep sliding it then dont read my post any further...however:
place the extender brackets into the channel at the desired location....you want to be able to rotate the extender when gate is open but not rotate it when the gate is closed..
then I intalled the channel end cap thingys (official term i think) the ones that require the cheesy little wrench to install...these thingys are used to keep items that are in the bottom channel from sliding out of the channels...i put one in front and one behind each extender bracket and tighten...unless someone has a wrench handy, they will not be able to slide the extender in to rotate the extender for easy lift off...
although you can lock the tailgate even in the open position to secure the straps...it only takes a razor blade to cut the straps and lift off the extender..so when not in use, rotate the extender and close the gate and then lock it.
So, although the extender can't slide toward the front of the bed (which actually is no big deal since the wheels wells are in the way most of the time, this seems to work pretty good.
I AM working on a way to lock the extender by possibly welding a nut to the bracketry and using a bolt with a small pad lock or small cable, to help keep the honest people honest and prevent the unauthorized rotation of the extender...i will post any success' or failures.