I like your idea of mounting the two "together". A Pi case could easily be attached to a CM15A using double-sided foam mounting tape. That could reduce my HA equipment shelving.... to just an out-of-the-way outlet somewhere.
I just used a wide elastic band but double sided tape would work as would a couple small nut and bolts through the two cases.
I plan to do that for my Off grid setup.
My Pi 3B+ arrived late this afternoon but I don't like the black case that came with it so I ordered a white one off Ebay for a buck. It should look nicer mounted to the CM15 as it is also thinner (at least it appears to be)
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I still plan to do a Pi Zero W add to my city CM15 this should fit inside the CM15.
I access my [Homeseer] pi [Hometroller] remotely... via the internet (from my laptop) when making changes. The on-line remote access for Pi severs is pretty typical (as I understand it).
I plan to do this as well for making changes with my off grid setup. I always take up my laptop anyways.
Although with my "regular" Pi... I use it directly with a WiFi mouse/keyboard... and my 46" TV serving as the monitor. But... thinking about this.... I could even add the security camera server.... and mount everything to the back of a TV.
this is how I had my Asus Eee Box (mini PC) hooked up behind the big screen TV.
It was running a weather station, HA with CM19 AHP and as a streaming box.
My son got us an Android TV box a few Christmases ago and my weather station died so my HA moved back to my development PC and I added Alexa control.
I loaded Linux to it to familiarize my self with the OS again.
Alexa control is the first thing I'll add to my PI most likely. I haven't decided what to do about streaming from it for the off grid place yet. Maybe just the web browser for now.