I upgraded my RepRap Guru I3 with a Bowden extruter. I couldn't find a good mount option for my extruder. I started with the Frame parts from https://www.thingiverse.com/thing:662820, specifically the Z-axis Top. Cut a side, extruded the top, notched the bottom and added/adjusted some holes. Now the extruder is properly mounted.
I'm using this extruder:
You'll need some additional HW specific to your frame:
2x 3mmx22mm Socket Cap Machine screws (attach adapter to frame)
2x 3mmx10mm Socket Cap Machine screws (attach extruder to adaptor)
2x 3mm Machine Nuts and washers
The screws that hold the Extruder to the adapter thread directly in to the PLA/ABS, no nut required.