equipment recommendation - How to prevent zoom-creep?

The Nikkor 18-200mm VR lens creeps until fully extended if pointed vertically down.

This is preventing me from using it on a rostrum camera.

Is there a piece of equipment or a simple (and non-destructive!) hack to hold the zoom in place?

I found a blog post that suggests using a rubber band half-overlapping the zoom ring, but that doesn't sound very secure. I'm really after something that will hold it exactly where I put it for a really long time.