I had a repro field laid flat and noticed it warped a little. Tim recommends they are best stored on their edge. That's how I've got them now anyway.
Tried a few standing up in their boxes but found they will warp a little over time so made a flat trolley and i just keep on stacking them.
No chance of warping or damage and they live under one of the machines so their out of the way.
And yes folks thats all NOS playfields in that photo with another 2 trolleys just the same,How many do you bloody need