In the house?
I always just used a stall.
You would be shocked how much renting a porta-potty is.
Making your riding area smaller when you add more horses to the mix does not sound like a practical way to go. Plus, the width of a free span building is what makes it expensive. You might not find it cost affective to pay for riding ring free span just to shoves stalls on the end of it.
Have you priced an indoor ring yet?
If you are asking about clay for footing in an indoor you need to contact someone who has experience doing ring footing. Clay is slippery when wet and hard as a rock when it is not wet. Not good footing. There is far more to indoor ring footing than just slapping down sand though.