Warning : I noticed that Ecobee changed the design of their smart sensor. I only could look at pictures of the new sensor and the back looks much more spherical, it probably won't be compatible with this wall mount.

I used the original wall mount thingy that came with the sensors and when I had to remove them to change batteries, 2 of them came with the screw out of the wall. I decided to design this wall mount so the sensors can be easily removed without having to use any force.

I printed this with the printer at my local library.

Edited 2019-11-06 : Same STL file, just uploaded new pictures. I bought an Ender 3 Pro and printed more holder for the sensors. I used Cura 4.3 with the default "Super Quality 0.12mm" profile for the Ender 3 printer with supports enabled. I used value PLA from 3D Printing Canada and it came out much better than what I could print at my local library.