I bought the ones made from pvc but have like a scrambles of threads bedded in to top.
This worked great until someone did a pee on the mat - and all the (clumping) litter that had accumulated in the wooly bit then set solid
Not too practical for wiping paws on either cos they just jumped out of the box to avoid touching it, spraying litter even further.
The one Susanne showed a link to looks promising though...