I have had good luck, by taking a shallow pan and fill with water, full sheet cake pan in my case.
place you carving, face down, over the pan, use small stickers, to hold over the pan.
Watch carefully, it does not take long, an hour or two, for the caving, to straighten out
Be careful, it will gain too much moisture.
Once flat, clamp, and hold, til dry, face to face, back to back.
I have used this method, many time, on large carved door panels
Will Williamson willcnc.com willmade.com
Williamson Lumber & Millwork Inc.