A german forum user [http://www.android-hilfe.de/thema/power-off-an-aus-taste-defekt.644199/page-2#post-9499367|DerGlonntaler] used this trick: [http://www.android-hilfe.de/attachments/img-20160118-wa0004-jpg.464432/|photo of the backside]
He used a piece of post-it paper to stick it on the place where the power button is.
The backcover of the phone produces more pressure with this trick and it should fix the problem.
I tried it myself and it's working so far :)