Hi Anthony,
I personally would have a concrete base, but it must be drainable. Then you can cover it with what ever substrate you need. I use a mixture of coarse grit sand and horticultural potting grit. It is easy to keep clean, I shovel it into a sieve and as the muck, castings and bits of bones etc. are left in the sieve I tranfer it into a bin bag.
Once or twice a week, give it a good raking which will also bring out any bits and pieces hidden. The potting grit also helps as -'rangle'for digestion.
The least amount of soil, or living matter the better, as these can cause bacterial infections etc.
Hope this helps, clive