The Yakima Button Down aero includes these universal mounts from the box, but these would not fit well on my oem bmw crossbars. So after work, I went and bought the Yakima Universal Mighty Mounts and these appear to fit a lot better. The mounts do not close up all the way and it looks like a lot of gap, but it is very tight and appears to be very secure. I believe it should be fine to use to transport my snowboard. I'm really glad I'm able to use the button down aero on both of my cars. Here are some pictures of the install on one crossbar.
2011 335i with M-Sport Performance package, Carbon Fiber interior trim, gloss black kidney grills. 18" Apex Arc-8, JB4.