You could also use PVC pipe with 2 end caps for motor mounts. The cap that will be mounted onto the fire wall drill 4 holes for your bolts.assembly the mount unglued ,attach the motor to it , stick it through the cowl. When you have the motor shaft in the center you could mark the end cap that`s touching the fire wall from the other side of the fire wall. or put a screw in the end cap from the back side to hold it in place. To do this you would have a small hole grill in the center of the fire wall where you think the motor mount would rest. Remove cowl and remove end cap with motor attached to it . Now you have a open end tube use a marker to mark out the 4 hole for your bolts. You can also put lightning holes in the side of the PVC tube.