We have a American Hophornbeam (Ostrya virginiana) tree in the side yard which is in good condition with the exception of a small triangular shaped cavity (approx. 3" wide at the ground and 5" high) at the root base. The problem is that the tree is in a fenced-in area that our dog is confined in occasionally. When he sees a chipmunk or mouse run into the cavity, he runs over a starts pawing at the bark around the cavity. Is there any way to safely seal the cavity without damaging the tree? I have the ability to fabricate a cedar "plug" which would just fit in the cavity to keep the critters from running in. Unless the dog sees something run into the cavity, he leaves the tree alone.