The Stow-Away lens cap holder project on Kickstarter offers an accessory for SLR cameras that attaches to the tripod mount and holds your lens caps (varying from 52mm to 72mm in size) while shooting:
The Stow-Away is an accessory for your camera that holds your lens cap under your camera. It screws into your standard tripod mount with a thumb screw, and can hold your caps sized 52mm, 55mm, 58mm, 62mm, 67mm, and 72mm. Designed to help you focus solely on your photography, Stow-Away provides the most convenient way to store your lens cap while shooting. I have used this prototype on my recent photo shoots and found that it makes shooting easier because I don't have to wonder where to put the lens cap, or where I put it last.