I have adapted my Nikkor 8mm f/2.8 circular fisheye and 16mm f/3.5 full-frame fisheye to my Leicas and other RF cameras.
I found a simple viewfinder on eBay for the circular fisheye - basically a door peephole on a shoe. It worked well enough. I don't do this often - but it does make it possible to shoot some specialty lenses on the Leica from time to time, rather than bringing an additional SLR.
I really want the Nikkor 7.5mm circular fisheye as it's much smaller but I haven't found one at a price I could stomach, yet. The penalty is speed though.
Here's one from my 8mm, shot with a Bessa L. The whole thing kinda fit in my large pocket...