Bracket for mounting RichRap's Greg's geared Bowden-extruder to RigidBot (or any 0.75 inch square-tube frame)
I used #8-32 screws, but you can use an M4 also (they're essentially the same diameter). The part is about 28.6mm, or 1.125 (1 and 1/8th) in width, so your screws must be longer than that, plus the thickness of the extruder plate (3/16th inch or 5mm).
I recommend a high wall-thickness and a high density, for strength. Print however you'd like aside from that.