I made this clamp knob using the customizer and http://www.thingiverse.com/thing:713300. I wanted the 1/4-20 nut to be trapped inside, so I modified the knob to do that.

Print with the larger surface down. The last layer of the nut trap is at 9mm, so edit your g-code by searching for G1 Z9 - it should find a line like G1 Z9.000 (this could be different, depending on your layer heights. I printed with .3 layer height, so Z9.000 was exact for me). On the line before this one, enter @pause. The print will now pause before printing the layer that starts closing the nut trap. While paused, lower the bed (G1 Z100) and drop a 1/4-20 nut into the knob and then hit resume.

I made these to go with these other pieces to use with my MP CNC.

MP CNC (amazing!) - http://www.thingiverse.com/thing:724999
T-slots for 1/4-20 carriage bolts - http://www.thingiverse.com/thing:876395
Hold down - http://www.thingiverse.com/thing:21057