I have an interesting very long tree root in my grass shaped like an alligator. I would like to add some garden art to my yard and paint the root like an alligator.

However, there is a three inch by six inch gap I would like to fill with some substance prior to painting. Would a concrete patch or something similar work, and more importantly, would it damage the tree (a scrub mulberry at the edge of the woods)? Any creative answers and advice will be appreciated by this grandmother and her granddaughters. Thank you for humoring me.