I'm not worried about it. It fits snug just right. I didn't apply a lot of pressure pressing it down and i pre-soaked it for a few hours before i put it in the tank.
I purchased it on amazon, but i had to return the first one i ordered because not one side was at least 18 inches.
Zoo Med Natural Cork Bark, Flat, Jumbo https://www.amazon.com/dp/B000QFUAFO/ref=cm_sw_r_udp_awd_jizEtb0YHRDWXA44