how to: Lock and unlock N4 by double tap

I looked high and low for a solution for this and am hoping you guys might have an answer.

I am trying to spare my power button from my chubby fingers. So thought hey double tapping to lock and unlock would be nice like on the Nokia phones. Below is what I found:

1) Custom launchers like apex, nova have gesture actions but nothing to lock or unlock a phone.

2) Most of the apps seem to be a hit or miss. Most use the proximity sensor. None of them seem to be a 100% thing

My phone is not rooted. Look forward to all the tips. Thanks in advance.