The Falling Viewfinder (preventing your viewfinder from falling off your rangefinder)

Preface:
I use a voigtlander R3M + 21mm f/4

I've been using this lens for about a year. Note that I usually like to supplement this lens with the 40mm f/1.4
The viewfinder that is supplied with this lens does not have the screwy thing (like on most flashguns) to lock in in place.

There has been one instance where the viewfinder has slipped out.
Now I'm afraid to leave my camera on my shoulder in case the viewfinder drops while I'm looking at oncoming traffic. (That is one example instance when I should be looking at something else and not my camera)
Do you have any solutions?
I also like to use off flash trigger on my rangefinder. So the attachment can't be permanent.

I have the same issue with a few viewfinders, so I use a bit of gaffer's tape to secure them. It's simple and doesn't look too bad. As long as you don't leave it on too long, it wont leave residue. The other thing I've tried is a piece of very thin polyester tape stuck to the bottom of the finder foot to make the fit more snug.

I use masking tape in the bottom of the viewfinder foot.

I've had the same problem in the past but wasn't smart enough to think of tape.
I used a paper clip bent in a "U" around the back of the finder and the front ends bent down to keep it from just sliding off.
Heat shrink tubing on the tips keep them from scratching.

When I shot RF I ran a piece of electrical tape over the top of the finder, down the side, and across the top plate for an inch or so. Folding over the last 1/4" makes a pull tab to make it easy to change finders. That worked fine for Leica finders with metal feet, and later, with plastic C/V finders I discovered that fleshing out the bottom of the foot with a strip of masking tape added sufficient traction.

I lost that very viewfinder off my Bessa-L into the Rhein (I think it was when a passing bicyclist bumped me), so I can confirm that the concern is realistic! The tape seems like an easy and robust solution, and I'll put it into action next time I use the 21mm.

-NT