I was netting nice ink coverage with: http://www.thingiverse.com/thing:13624 but I started having issues with hardcore reversals that would send the marker shaft rotating into position so that the z stage couldn't raise further due to the Sharpies hitting the bot's ceiling. So,,, I increased the shaft's length at the bottom to clear the MK6 extruder stepper and fan with the lowest mounted Sharpie (leaves the most room for vertical movement) and added a latching brace that utilizes the MK6 top right M3 bolt so that reversal torque movement is eliminated.
I took the original stl that Bryan Duxbury uploaded and imported it into SketchUp (also attached below) and made my mods. Let me know if you have any issues with printing or functionality. As you can probably guess this was only tested on a factory MK6 setup and I have no idea if previous extruder builds will work.
This was printed and used on:
TOM # 4890
ABP with aluminum plate topper and titanium belt covered in kapton
Remove top right M3 stepper bolt from extruder
Slide the Mega Felt Marker Topper over your 1.75MM filament
Mount filament into extruder
Insert M3 bolt back into the top right extruder stepper hole and tighten
Go nutty bonkers with color!