OK....here goes my top secret data......
I started with 7/8" diameter cold rolled steel round stock. In a lathe I first drilled a 9/16 diameter hole 2-3" deep. I then bored the hole to a slip fit over the Harris factory knob. On both of my bipods, that was .576 diameter. This fit is not critical for function, but I'm kinda picky about fits. A 37/64
(.578) diameter drill would also work fine.
In my case, I used a part-off tool on the lathe to cut the "rings" off .400 long. If you were using a piece of pipe you could also saw off a "ring."
The last step is to drill and tap a 1/4-20 hole through the side of the ring. You can adjust the length of the bolt to suit your needs. I kept it fairly short to keep from getting wrapped up with my sling. I used a 1 1/4 long bolt.
To set up the lock, crank the knob on the bipod around until it locks. Slip the ring over the knob. Swing the bolt around to the left until it is against the edge of the bipod and lock the bolt. Now when you swing the bolt back to the right, the bipod will pivot freely.
You can either paint or cold blue the ring and you are ready to go.
Let me know if you have any questions.