I use a Velcro strap (Home Depot) around the legs. Secure the MP transceiver with some additional Velcro and/or a plastic tie. Shown here on my P3P, works the same on my P4. I keep the antenna roughly centers between the legs to minimize chance of interference with the quad's leg electronics.
Here's how I mounted mine on my P4. I fabricated the bracket and installed the Marco Polo tag. Haven't been able to spend much air time (bad weather) to see if it effects any of the flight aspects yet.