That cheap plastic box I ordered on Amazon took quite some time to be delivered... until I realized it was shipped from China. 

The size is just perfect. I cut a few holes, one for plugging in an USB C cable (to allow direct reprogramming without having to take everything out of the box), one to pass the cable from the air conditioner and I added a small light pipe so the microcontroller LEDs could be seen. All was fixed using my best friend number 2, hot glue.

The final result looks like this (case hold with blu tack):

I may redo the cable that connects to the air conditioner motherboard but for the time being, I'm happy with how it looks.