Instead of caps, I just bought some 4x4x2 white boxes from a local shipping store and store the lens and the lensboard (with some silica gel) in the box. I have a pair of Beseler enlargers and these boxes protect my lenses from dust and scratches, and keep them attached to the lens board for easy swapping. None of the glass elements of any of my enlarging lenses extend beyond the barrels that surround them, so they never come into contact with the top or bottom of the box. And since the box and the lens board fit so snuggly together, they don't rattle around.