I put a little piece of "self-adhesiv velvet" (ok, I don't know the name, I hope you can understand what I mean) under the dioptric lever, just to have it a bit more sticky. If you wear eye-glasses, I'd suggest also to put a rubber ring on the viewfinder, just for protection, and (just in case of light-leaks) a little strip of the same self-adhesiv velvet into the groove in which you insert the removable back.
Enjoy your new toy!!
Franco