Do not use velcro because the Naza has dampening in the case, hence why they want it hard mounted. If you velcro it wobble to where you can not get rid of the wobble and any vibs from the props frames etc could make it worse.
That is what you want to use and nothing else. Make sure that the Naza is parallel to the center line of you build and in the center. If it is angled or canted, it will cause you problems.