Grass or Wood, when almost anything can absorb moisture, it typically gets bigger. See below for more on this...
The clamps have plastic feet.
Probably considered cross posting, but reliant to the grass vs wood hangup 🙂
As a highly scientific test, I took two bamboo board cut offs, that measured almost exactly the same.
1 was 1.080 and the other 1.082...
Put one in a cup of water, nearly 24 hours later it measures 1.090 So, it grew 8 thou when soaking wet. Dont imagine it could get much wetter.
In comparison the second block was placed on a warm surface (Laptop power supply) for almost 24 hours and it now measures 1.076 or a reduction of 4 thou...
Even if I take the difference, of the two, add them its .012
Since the samples were almost 1" to begin, and my widest panel is 15" I multiply the .012 by 15 and I get a whopping .180 or a hair under 3/16"
Since I don't plan on submerging these anytime soon, Id say that's pretty dang stable.
All in all, I still need to make some allowances, but clearly a heavy 16th to an eight of space per edge should do it...
That said, adding 3/4" bamboo board as a skin say to an aluminum frame that wont move, or another wood that moves more, allowances need to be made...