I had this idea about two days ago — make a lens cap out of the same material used for an ExpoDisc and you can leave it on all the time.
Turns out it has already been done. [via dps]
Rather begs a DIY version though doesn’t it? I have a cheap 3rd party extra lens cap (bought at great expense in Venice just down from the Rialto when I thought I’d lost my original Nikon cap), which I could merge with a Pringles can lid with or without a coffee filter.