Cheap and cheerful, it does the job.
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Back in the day when I owned a 21mm f/2.8 Elmarit ASPH (they were affordable three decades ago) I used the genuine Leica 21mm finder. That ran some $250 back then and it was pretty awful, with heavy barrel distortion. Today you can spend $1000 on a new one or $400 on the Zeiss (Cosina) version. Reviews suggest that the Zeiss is every bit as awful but you leave with $600 in your pocketbook.
Used beater Leica finders can be found for $300. Voigtländer used to make a couple of versions – a clunky and ugly plastic one (also sold under the Ricoh label) and a rather elegant metal torpedo type which included frames for a 25mm lens and came with a satin chrome or black finish. It’s so-so by all accounts and used ones crop up occasionally for $150 or so.
Why am I writing this? Well, when I reviewed my recently acquired 21mm f/3.5 Voigtländer VM Color-Skopar lens I suggested I would use live view to chimp the rear LCD screen as a composition aid. Well, forget that. It’s a poor experience. You hold the Leica like an iPhone, looking like a real twit, and the display is seldom bright enough to see much in bright light even when set to maximum brightness. So I shopped around and came across this dirt cheap plastic finder which ran me $26.40 shipped from a US seller on eBay:

At that price it’s not a great risk to try.
It’s not going to win any prizes for looks but, amazingly, despite the high barrel distortion (what else is new?) it delineates the field of view of the 21mm lens accurately side to side and top to bottom. It’s also very bright, has no frame lines, defeated all my efforts to photograph an image through it and weighs just 5 grams/0.2 ounces. And it’s a nice tight fit in the M10’s accessory shoe and will not fall out even if nudged. There is no lock on the foot and none is needed. It’s also tiny and fits nicely in my shoulder bag. Sharpness drops off to the sides of the view but not enough to make composition difficult. It’s also set back too far in the accessory shoe meaning that you get a part of the camera’s top plate in the view, but it’s nothing you cannot work around. And it beats chimping.

Tiny.
The finder protrudes slightly behind the top plate but that’s not an issue in use. I may eventually take a hacksaw to the foot to mount it further forward, more to avoid seeing the top plate of the camera in the view than anything else.

Rear protrusion.
I have no difficulty using it with eyeglasses and the plastic should preclude scratches to my eyewear.
Try it. You might like it.