I know I've posted this several times over the last 5+ years, but here goes; I take the stock bar end off, epoxy or glue the bar end pieces together so they turn as one and use a 1" faucet washer to go inside the grip and a 1.25" to match up with the throttle grip. You may not need the 1" depending on where your throttle sits on the bar. This creates a friction lock you adjust by turning the bar end. On long trips I set mine to just enough drag to hold the throttle. I'm comfortable with this as I've had bikes from the '40s and '50s that didn't have return springs and Super Glides that came stock with friction locks.
Yes, I know this makes me a cheap bastid, but try as I may, I can't think of one reason to spend $100+ to replace something that already works fine for me. YMMV