Adapt ground plane size to a mesh in gazebo

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Hi!

I want to add a ground plane to a cave given by a mesh. However I have only found how to add a ground plane defining its size by its width x length.

I would like to limit the ground plane just to the inside of the cave, but the cave doesn't have a regular form.

I know that I could set it on all the environment as this wouldn't affect my simulation, but since I am using a color for the plane to match the color of the cave, all my environment turns to that color, being a little bit weird.

Thank you for your help!