How to mount ZWave switch on stone wall - either retrofit in metal box or better?

I have a light switch in a surface mounted 4" metal box in the basement. I replaced the switch with a Decora-style Zwave one in the single-device metal cover. I thought it was worth attempting because the antenna is behind the plastic paddle, exposed to the front, and there are other ZWave devices in the room. It works intermittently but not acceptably.

I’d like to avoid replacing the entire box with a plastic one: a) because I’m lazy b) because it’s on a rough stone wall in a high traffic location, I’m worried a plastic box might become damaged.

So I’m looking at a 4" single-device PVC mud ring as a compromise. It would go over the metal box and expand the exposure of the antenna through plastic from just the paddle to the entire 4" face. It might not work but it costs $2 and is worth a try. It’s very ugly .. metal box, blue plastic cover, white plastic paddle.

Any better ideas?

1. What would be a good short cut without replacing the whole box?
2. If I remove the whole box and "do this right", what is the best way to mount a Z-wave switch on a solid stone wall, in a manner that is to code and looks at least slightly attractive?