When you have a lot of equipment, dragging it everywhere with you can be a pain, especially when the weather's bad. Sure, you can put it in a camera bag, but a lot of them don't protect your stuff from water, and waterproof cases can get expensive.
Rather than buying a case, with two plastic bottles, some foam (or other soft material), a zipper, and some dental floss, you can make your own like Instructables user marcellahella.
After cutting the bottles, marcellahella used a safety pin to make holes around the edges. Using dental floss for extra strength, he sewed on the zipper, then cut a few pieces of foam to protect the lens inside the bottle.
This next step is optional. If you only want a protective case, you're done. But if you want it to be waterproof, you'll need to cut a piece of inner tube and sew it into a ring that fits over the zipper.
How well this protects your lens depends on how tight you make it. You should be fine in rain, snow, and other adverse conditions, but I personally wouldn't recommend submerging the case unless you're positive you made it tight enough. But why would you even try? Lenses aren't cheap, you know.
If you don't care for the look of this project, there are tons of ways to customize it. You could paint the bottle, use different colored thread, or, for shorter lenses, why not make it into a cute Pacman-style bottle?
Check out marcellahella's tutorial for the full instructions and more photos.