I just returned from a week of shooting in Alaska on a workshop, and we had some rain on more than one occasion. Many of the participants had long telephoto lens like the Nikon 200-500mm. I was carrying a 300mm F2.8, also a hefty lens. When you are walking through the marsh hand carrying these lenses, it is easy when wet to have the lens slip right out of your hand. If possible I will try to mount the lens on a tripod and carry it on my shoulder. But sometimes it is better to take it off the tripod and put it in a pack or hand carry it if the terrain is really steep. How to get a better grip on that expensive glass?
Just go to your local skateboard shop and buy some inexpensive grip tape. Grip tape is similar to sandpaper, but with a sticky glue that attachs the grip tape to skateboard decks…or the top of the lens mount bracket (think handle) that many long lenses have. The image above is from my 600mm F4. I just applied the grip tape to the top part of the bracket. Now that lens practically sticks in my hand!