Need somewhere to store your valuable DSLR lens and can't afford a custom bag? You can make a simple case using two plastic bottles, a sheet of foam and a zipper.
Instructables user made her case by cutting the ends from two plastic bottles and sewing them together with a zipper using dental floss and a needle. Cut two circles and a rectangular sheet of foam and glue them inside the bottles.
This makes a basic lens case that offers some protection from falls and water; you might want to consider waterproofing around the seams if you expect it to get wet.