This is a derivative of http://www.thingiverse.com/thing:261017 I've changed the size of the two recessed holes to match 3mm bolts so it can be mounted from under the extrusions using two button head M3x6mm screws, this means you don't have to disassemble the top of the Kossel to insert the additional nuts. I've also added two recesses on either side to give some clearance for the extruder wires while inserting/removing the EZStruder.