Here is how I prepared my Summicron R 50/2 work fully with the Speed Booster.
Disclaimer: I do not recommend that anyone do this and if they do, I bear no responsibility!
First, the rear was
generously taped off with masking tape (used for painting). I found the hood very useful in holding the lens securely.
Untitled by
Vivek Iyer, on Flickr
Then using a key file, the rear protruding shroud was filed off,
gently and without touching any of the masking tape.
Untitled by
Vivek Iyer, on Flickr
NEX-6, Speed Booster, Canon EF 50/1.8 II, Effective f/1.3
After removing the masking tape, the rear of the lens was held close to a vacuum cleaner hose to make sure there are no metal shavings left (about 10mins) and the rear element cleaned to remove any tape residues. Infinity works fine with my sample using the a cheap EOS to Leica R adapter.