Wanted to see if anyone here has experience with this holder
http://www.betterscanning.com/scanning/vb_mfholder.html

I have this one and the Variable Height Mounting Station (the larger one that can take up to 5x7 or full plate but not 810)

I'm actually on my second MF holder as my first one warped after about a year. When that happened Doug was very gracious, took it back and sent me a new one. All good, went back to scanning.

Now after a little less than a year the 2nd holder is warping too. I asked Doug, he told me the one I sent back is now flat as a pancake in his storage so it must be my environment causing it. This is very possible but I can't see what could be causing it.

I've had the larger VHMS holder for a much longer time and store it the same way in the same place, no warping at all.

The warping on the MF holders both started in winter time (I'm in NYC) but they aren't stored near a heater or in the sun, the temp is relatively constant and it's dark, the surface is flat.
Since the new one warped I've moved it to a different location and kept it with a large flat weight on top for several weeks - no improvement
The second holder isn't as badly warped as the first so I'm still able to use it for 6x6 or 6x7 scans without problem

In comparing the MF holder with the larger VH Mounting Station I noticed the plastic used is much softer in the MF and wonder if this is the issue

NYC winters aren't especially cold, dry, wet, etc, so I'm wondering what could be happening and how I can solve this issue.