The Port would get hot on its top surface if anything was stacked on it. Its main board is just inside. The Port has no inbuilt ventilation and is designed to radiate heat from the top. 

You could try putting the Port on its side, with space next to what was the original top surface. Check for heat though after it’s been operating for a few hours. 

I suspected stacking would not be a good idea. Will try placing the port on its edge. 
 

thanks

I tried stacking a pair of Play 1s, heat was not an issue but there was radio interference that  Sonos could see in a diagnostic I submitted.

I was able to relocate them far enough apart to dodge that issue. If I hadn’t been able to do that I’d have considered a suggestion to wire one to the other and disable the radio in one of them.

If you see radio interference you could probably wire them together and disable the radio in the One.

Submitting a diagnostic and requesting Sonos review it (they will want the diagnostic number) can tell you if you have an issue.