You need to do something to hold the battery and rx in place. If the battery is in the tank compartment that is usually good enough with some HD foam. The rx as you say is often loose in the fuz under the wing, can you get a couple of cable ties around something to hold it in place or glue in a couple of blocks with open screw in eyes and use a couple of light elastic bands to hold the rx down.

If the battery is in the same open open compartment you MUST take steps to make sure it does not move around with either bands or ties.