Where are you planning on placing the round pen?
What is currently the "footing" now?
If it is grass...leave it.
I would not put anything different in my round pen for footing,
A few times of use and you will have a track from using the edge more often..
If your location has longer grass in it...turn the horse loose and let it graze it down to nothingness..
Just using the round pen will break down any ground with the exception of concrete or blacktop, hopefully not surfaces dealt with.
Remember anything you add you will then be having to rake and smooth periodically, filling in usage ruts...
The more material the deeper the footing, = more torque and harder the ground your horse must move through = leg strain.
Be cautious how much depth you create with the best of intentions.