This is my design for a direct drive extruder. I included the Sketchup file if you want to modify it for a Bowden.

It uses the following components:

• Printed extruder body
• Printed idler
• Kysan 1040230 geared stepper motor
• Hyena 8mm hobbed pinion
• 3x M3x10 bolts (for mounting to motor)
• 1x M3x50 bolt (idler tension adjustment)
• 1x M3 nut (idler tension)
• 1x spring
• 1x M4x20 bolt (bearing shaft)
• 1x 624ZZ bearing (idler bearing)
• 2x M4 washer (for bearing)
• 1x M3x20 bolt (idle pivot point)

As you can see from the photos and supply list; this is a minimal design. You can use M4 bolts and nuts to mount it. It mounts to a standard aluminum mounting plate.