Here's how I've been storing my Mirco AFM for a year now about two inches from the wall hanging in some old side rails attached to it, I had to drill some extra holes though.
I'm certainly no expert and this will probably be the only playfield storage I'll do but no warping so far.
Temperaure is roughly about 17-20°C and 40% humidity most of the year, it will hang for a few more years until I find the time for a swap.
20180724_204446 (resized).jpg