To protect your Leica M lenses, FotodioX has you covered with their Plastic Rear Lens Cap. This lens cap protects your Leica lens's rear element and is made from durable high-impact plastic. The cap is crush resistant and precision-made to fit the Leica M mount. Keep your Leica M lens glass safe and scratch free when in storage or not in use.
It fits but easily slides off the lens. It does not fit as standard Leica M reach caps. There is no snug attachment.
TL;DR: They're fine. They get the job done. These lens caps fit my Minolta M-Rokkor lenses somewhat loosely. I was able to mitigate that by adding a little bit of gaff tape to the surface the would be pushing up against the body and/or the outer ring of the lens cap. It's ... fine. They fit, they're light, and they're cheap. They fit my M-Hexanon 28mm just fine, and rattle around on my 50mm Canon LTM with a LLL adapter. I'm sure there are varying amounts of fit and snug-ness, depending on the lens.