vertol - the cheapest way is to use a PVC pipe cut to the length that you need and that is put at the back of the motor to keep the correct space. Then you use the long bolts to mount the motor on. What I do is to make my own mount using aluminium rods, you get the ID (inner diameter) and OD (outer diameter) sorted out and have them in different lengths. Its the same as what you can buy from Aircraft World. The Hyperion style ones. Hope the pictures help you to get an idea ...