There are many opionions on this I didn't use the tray. I have my Rx mounted on the botom plate with the 3g unit sensor mouted on the anti-rotation bracket. To date I've had no issues with this locations. The heli flys great. no wag or other issues. I fly light 3d (still learning ). As far as the 3g sensor goes I used 1 of the included gyro pads under the sensor with great success. I don't know what your running or planning to run for batteries, I'm running the Gens ace 5300mah 6s battieries from hobbyparts. They are rated at 25c which I've been told by a friend who also uses them that this is a conservative rateing, & they can be charged at a 5c rate. At a cost of $95.00 shipped very cost effective.
Hope this info helps. Keep us posted on how you make out.