Watch the heat and sun. Depending on where you are, the heat might be high enough to warp the mat. I'd lay it down with the curled side down and put weights on it.
Here's info from Quilter's Rule on the Mega Mat
https://quiltersrule.com/instruction...are-Guide1.pdf It specifically says not to expose it to direct sunlight for long periods or excessive heat. I'd be careful how you try to flatten it.