If you are like me, you probably bought a lens pouch for your spare lens when you first started in photography. I used mine for a couple weeks and it sat in my photography room on a shelf for years after that. I found it much easier to just let my lenses stay loose in my camera bags as they fit tightly enough in the divided sections. That all changed when I started using the Peak Design Range Pouch. This is by far the best lens pouch I have ever used. One of the great things about this pouch from Peak Design is that it has a separate divided inside that can fold down and allows you to store two lenses on top of each other. This is perfect for carrying two small lenses or a bigger lens and a pancake lens, which is what I do. I love it because it adds extra protection to both those extra lenses and allows me to store them both in one divided area of my backpack and are easily removed and kept in the Range Pouch when I want to carry spare lenses on my body or switch them to a new bag.
Another thing I like that really helps with carrying different sized lenses is the velcro top to the pouch that can connect wherever you want so the pouch can be almost whatever size you want. This is great to keep your lenses tight and secure and as compact as possible when you use a smaller lens.