Whats the name from your dog ?
I think I may go with a lined one (like those hard plastic ones, preformed)
Really love the pond/ mud hole idea, nice that they can go in and out at will and wallow around. When I think of planning for a larger area next summer, I keep going over ideas for a pond, I think I may go with a lined one (like those hard plastic ones, preformed), maybe even seal painted concrete. But then I'd have to put in a fountain, or waterfall to keep it running and not get stagnant, and clean it once a month. My soil here is so sandy I think mine would drain out so fast I could't even keep it for a few hours in an open hole (except maybe in rainy season). Anyway...great addition to their world