I know this may sound weird, but I would recommend just going with the basic Sonoff switch and skip the MySensors part.
I build a lamp controlled by MySensors a long time ago, has external Tact button for on/off. Works great, took a while to build and I have an ugly box screwed to my lamp now that houses it all.
I wanted to build another, but didn't want it be be aesthetically displeasing. I ended up using these (https://www.itead.cc/sonoff-wifi-wireless-switch.html) and they are everything in one small package. You have wifi, a tact button for manual on/off, and it will report to any home automation system using MQTT after flashing some customer firmware (https://github.com/arendst/Sonoff-Tasmota). Just cut the power cable, screw into appropriate terminals and it's all good to go.
As much as I love MySensors; for this type of project I think there is a better solution.