I ve used stoc board inside dog cabins that stuffs alive, it swells with heat, I'm talking a warm day then contracts in the cold, very strong though, only pictures I could find, you can fold it in half, wack it with a hammer, it won't break, I doubt a dog could scratch through it which is why I used it
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If I made it float in the frame do you reckon 5mm would be enough expansion room over 500mm which would most likely be the height and width of the box by about 1500mm depth