What materials should I used for my dog run? (Somewhat budge friendly)

Hi! We’re looking to make a dog run about a 18x18 ft space for our dog to access at all times. Originally I wanted to used turf so it would be easy to clean and not peastone which I’ve heard hurts their feet and is also a tad messy.

The ground is a bit unlevel so I’m worried that the cost of leveling for turf would not be in-budget.

Any ideas for what we could do instead? Grass wouldn’t to be ideal since his pee is like acid and ruins grass. We had mulch at our last house but he tracked that into the house constantly which was a pain.