D Ben wrote:
Use gaffer tape. It's been successful on my Canon 100-400, when shooting moon photos, with the lens at an acute angle. Consider leaving a tab on one end for easy removal, and a Ziploc bag to store it in for future use.
That is one of two extending lenses with a locking collar that can be locked at any point of the zoom by twisting the collar, even though it takes two hands to do it.