fwiw, things I've done:

- on a few electrics, just double stick tape
- on some foamie combat planes, cut a hole in the foam the size of the rx, and shove it in there.
- on some profile combat planes, wrap in thin padding (~1.8th" thick, less than 1/4") and cable tie in place
- on some larger glow planes, wrap in lots of foam, cable tie in place. Or secure with rubber bands, or glue in a bit of wood to hold it in (I like to be able to remove gear, so this is not prefered.

I've never had an RX just die. I've broken plenty of them through various mis-adventures. (combat mostly).

I *think* you'll be fine. But I still can't bring myself to recommend you not get some padding of some sort in there if you can.

Taken another way, if a beginner showed up at my field with an RX mounted as you describe, I wouldn't ground the plane. I would check to make sure it's not going to come loose in flight and bang around or jam a servo. And I'd see if I could offer a specific plan to better mount the RX.

How's that for lots of information with out actually answering the question directly?