Sand definately works.
We are always building or plastering something, and constantly need a pile of sand, the 7 cats we have use it as a loo, then we cannot plaster. So we now have to keep our plastering sand under cover.
So try a little sand pit, but don't put it too near the veg patch, put it near where they sleep, or they will just poo in both of them.
You could try putting a bag of cat litter on top of the sand if they were ever used to it, as the smell will attract them.
Julie