Get a good big container, nice and deep, and then there are several options. You don’t need to fill the container to the top, but allow a good 5-6 inches of whatever you’re using, plus an extra 12” minimum above the sand so that when they stand up to shake it off, more of it stays in the container rather than spreading out into the run. If you have any spare ordinary garden soil with not too many stones, that will be fine, but it may not be possible to poach this from your garden borders without damage to the plants. I use play sand, comes in bags from garden stores, and mix in what earth I may have, such as the leftovers from flower pots when the plants have finished. Probably it’s not so good to buy compost intended for growing new plants or seeds as it will have quite a lot of added fertiliser in it. Whatever you use it needs to be dry. Usually I have to spread it out on a tarpaulin in the sun, ( when we have any!) to dry it out.