Look around for a reasonably (hah!) Benser Baseplate if you can find one. Benser mjade Leica M accessories back in the 1960's which were actually listed in the Leitz catalog. They were best known for their leather cases with interchangeable compartments. The Baseplate fit over the camera's baseplate, attaching at one end via the protrusion on the opposite end of the camera from the one the regular baseplate uses, the other end secured by a screw in the tripod socket.
It has two lens bayonets, actual made-by-Leitz Leica M flanges with positive lock buttons to secure the lenses. The finish on the rest of the unit is matte chrome finnish that really does match the rest of your camera. It's sturdy enough to hold a 90mm Summicron or my heavy-as-hell 85/2 Nikkor, and probably even an 85/1.5, together with a Noctilux if your shoulder and strap can handle the weight.
If you carry only one extra lens you can secure it, then put the one you were using on the 'plate before mounting the second lens on the camera.
Leitz used to make a 7.5mm bayonet to bayonet extension tube. If you can locate a couple, maybe three, of those, a 21mm lens would be no problem either. I picked up a few at five bucks apiece when Leitz closed out their stock of Visoflex related items. The 'plate listed for $39 Way Back When, but I picked mine up used several years later for less. I even still had the original fitted box a few years ago when somebody made me a fantastic offer that I just couldn't refuse. They've turned up on rare occasion on the Auction Site, and since the past two generations of Leica folk have probably never heard the name Benser not many people run a search for them.