The mk7 is a marvel of simplicity but I'm still partial to ball bearing idler extruders.
I just don't love having that delrin plunger dragging on the filament and I like to be able to quickly release the tension so that you can pull the filament out and replace it easily without having to run the stepper.
6/26/12 - I just uploaded a new experimental version with a recess for a nut for the tensioner. I haven't printed it as I'm out of town at the moment but feel free to give it a try. It's called mk7-lever-drive-tension-nut.stl (It should be the last file). The lever is unchanged.
Either print out the parts separately or print out one of the plates (with or without the quick release).
Remove the support from the lever (where the bearing goes, it's just two thin walled cylinders)
Remove the support from the underside of the body.
Remove the support from the quick release (two cylinders again)
If you have a 3mm tap, you might as well run it through the holes for the tensioner and the bearing.
Install a 623 (or R2) bearing (3mmx10mmx4mm) in the arm with an 3mm x 10mm countersunk screw
Put the pin for the quick release in the quick release and install the screw (3mm x 20mm)
Snap the lever onto the drive and install the tensioner screw or quick release.
Install the drive the same way you would the normal mk7 block. Tighten the tensioner screw until the stepper misses steps when you hold onto the filament (as opposed to just slipping).