I needed a secure way of mounting my quads to the outside of my transmitter case, so I made a small holder. The holder is done so that standard rubber hair bands can be used to strap down the quad. Two zmr250 quads fit on the outside of a standard size aluminum case.
On my own bag I secured the holders using hot glue which can easily be removed using alcohol. This means that I can remove it without trace if I later change quad type or use the case for something else.
The holder is made so that allen m3 screw heads can be used under the quad without being in contact with the surface. If you have something sticking out more than 3 mm from the bottom of your quad, you need to place some kind of spacer under the mount. Or you could use liberal amounts of hot glue to raise the mount slightly.