Part 3: Further modifications
Since the large telephoto lenses commonly have only one tripod hole, it is possible for the entire assembly to rotate on that axis. This has the potential to spoil time exposure photos.
The simplest modification is to add sheet rubber to the mounting surface. Sheet rubber can be obtained at the hardware store. Bicycle shops often have discarded inner tubes in the repair shops. The thickness is perfect.
The rubber is glued down with rubber cement. Allow 24 hours for it to completely set.
Additional stability can be had by adding a counter-rotating pin. This mod is usually specific to the lens. As it turns out, it fits both the Canon 400mm and 600mm lenses, and does not interfere with mounting the smaller lenses.
The pin is held with an additional plate mounted in the channel.
Assembled as above, the lens no longer slips, even if considerable force is applied.