One way that I've found helps hold things in place is to use gravity as another hand. Tip the plane so that it's tail is pointed down, then you can set the motor and mount right on the firewall and gravity will hold it in place. Of course, you need to have it low enough so that you can see everything and figuring out how to hold it might be a little tricky, depending on the plane and its size. Don't know if that might work for you but until we grow another arm, it's the easiest way to get one without having to pay for liquid refreshments for a bunch of stinky friends.