Our ark is built as a lean-to in a corner made by 2 stone walls, and the floor is outdoor grade plywood fixed on top of 2 pallets. The soil here is heavy clay and holds a lot of surface water when it rains, so I thought a raised floor was essential to keep them dry and warm.
We got our 2 piggies as weaners and they've kept their bed spotless, I've been in to check a few times and I could honestly sleep in there myself! I've given them extra straw from time to time but quite often they chuck it out, as if they think there's already plenty in there. They've made a toilet area in part of the pen furthest from their ark and I've not poo-picked, but will clean it up after they've gone.
Overall, I've found them to be the least trouble of all our livestock (sheep, poultry and ponies!).