A rubber horse stall matt.
They are made of rubber and are large, a dog can't dig through them. You can get one of these in 3/4 inch or 1/2 inch. You buy them at any horse feed supply store or you can get them at a agricultural store for livestock.
I use them in our trailer for our horses. If a horse can't paw through them a dog can't eitehr and they are REALLY heavy so they will not move easily.