I've been looking for a cheap around-the-house solution to losing my 49mm Minolta lens cap, and found out that if I cut the top off of the flip-up lid of the new (hideous) style of Folger's Instant 3 oz Coffee, it fits well enough. Probably a little piece of tape or felt would make it more snug, but it does work as is. Then I discovered that because it has 2 different-sized lips on the under side it ALSO fits my 58mm Canon lens, and it fits that one superbly and snugly, snapping into place. After googling I learned that other Folgers lids have been used for lens caps for many years (didn't know that). But this is a fairly new style of lid, shown below. If you don't like red, get the green decaf versions.
If you hate instant coffee, you can send the contents to me because I actually drink the stuff.
The larger 16oz size will probably fit a couple of larger lenses. That one is just a hair too small for my Apo-Nikkor 480mm