I tried ninjaflex without the adaptions being stubborn, and I got about 3 minutes into a print after some careful setup, then it came round the wheel the same as everyone else, guess you cant cheat the odds

The real cure is to replace the base of the aluminium part, with a redesigned aluminimum part that comes up to the drive wheel the same as the plastic part does the job of, possibly extending the top lever part down also but a bit of testing would show if thats needed too. Maybe thats something printrbot themselves could take onboard since at manufacture time it should be a case of using a different cad drawing for the next version (ninjaflex friendly 1.x

) , and I'll take this route myself when I have spare time. I might extend the two parts out left to form a handle like the thingiverse extruder handle does (which is great, if you dont have it already
http://www.thingiverse.com/thing:507944 )
But... I printed off the adafruit part, and it didn't fit my simple metal either, possibly the parts there have changed slightly during production who knows. But, after printing 4 off, and going through a process of shaving each one until it fitted, I finally got one that fitted well and does the job.