Here’s a thrifty DIY idea courtesy of Netherlands-based photo enthusiast Rob Gipman. After getting rain on his lens a while ago, Gipman decided to take an unwanted piece of water-resistant fleece and make a do-it-yourself covering for his Canon 100-400mm lens.
Making a custom cover allows you to come up with a perfect fit for longer telephoto lenses that generic one-size-fits-all covers might not protect adequately. If you’d like some water-resistant fleece, Amazon is selling waterproof fleece blankets for $19.
Although Gipman’s was already double-sided and water resistant, he decided to further waterproof the covering by applying a water-repellant spray (you can use 3M’s Scotchgard for this).
The beauty of Gipman’s cover is that the softness of the material allows the push-pull zoom of the lens to be fully functional while it’s being protected. (Using this type of thicker cover for a rotate-to-zoom lens might be a bit more tricky.)