Being unsatisfied nor with the artificial rock I created nor with the tree without anything filling the space between the “legs” and inspired yet again with JanTam blog I decided to fill in the hole with real rocks which I glue together.
This is the beginning stage:

Collected some rocks from around of the same type and different size with one particular decided to cover the front:

Filled in the hole from all sides as tightly as I could without damaging the bark:

Did some grinding of the biggest stone with Dremel like tool so it can fit as close as it could to the front and also fill in the gap inside:

To the stones from the inside I applied epoxy glue being careful not to glue the tree itself:

Pressed well and waited some minutes:

The end result can be seen here.

Perhaps the stone type I used should have been a little bit darker. This one blends with the bark. I thought that would be a good idea but maybe it wasn’t. Anyway, I am OK with the end state, much better than it was anyway 🙂