I've been wanting to find a solution for locking (2) stacked rotopax with the locking handle without losing the security of the lock... in other words, keep people from being able to spin both the handle and 2nd pack off the base. From all my searching, I couldn't find a solution so I came up with the below.

I ordered the longer extension mount hoping that if I remove the lower piece, I can thread the extension all the way down so it lines up with the existing mount, thereby locking the extension to the lower pax and the upper pax to the extension - this did not work as the threads on the locking handle wouldn't reach. So, I put the extension back together and noticed that dimples on the lower piece would work great for a counter sunk screw. So I lined up the two halves of the extension mount, drilled through one dimple into the larger piece and tapped it. I then drove a stainless machine screw - in my case I used an M6-1.0 socket cap flat head screw (note: this pitch on this screw head was too steep to sit in the dimple flush, but sits flush enough to work). See pictures below.

The screw installed:


The extension mount installed:


The first pax installed - note this pax keeps the extension from spinning:


Both pax mounted and locked up - upper pax and handle cannot be spun off: