I made this adapter to allow the Ecobee to be installed in a North American type of electrical box. Most of the thermostats installed in newer houses have this type of box behind them, Personally, I didn't feel like patching up the hole in the wall just for a thermostat installation.
I've added some pictures of an electrical box and the Ecobee mounting plate. The screws shown in the Ecobee mounting plate picture go through the small holes in this adapter. The adapter mounts to the two holes in the plastic electrical box.
I hope you find this useful!
Printed mine in ABS with 90% infill. Takes about 45 minutes. Mild warping isn't really a problem for the finished product. Should also work well with PLA.