I found these online, they’re for stereo earbuds etc. but I measured up and I think that they’ll make nice small hard cases for my Olympus 9 mm and 15 mm body cap lenses. They’re available from Amazon individually or as a two coloured red and blue twin pack which is what I’ve ordered to make them readily identifiable. The lenses are plastic and I don’t think they’d stand up to hard knocks. As they have a mesh pocket they could also be useful as filter cases for filters up to about 70 mm in diameter or possibly slightly smaller.