I have on in one of the hole of my box. It's a good mat, but it does have some down falls...
1. They only come in two sizes, neither of which fit my holes properly, even when cut.
2. The holes they have in them are large. A little too large for my liking. I'd rather have more, smaller holes.
3. The drainage is poor. There is no lift or raise between the mat the floor of the box, so though the water may not be sitting on the mat, it doesn't dry/air out very well. the box can stay muddy if one doesn't remove the mat immediately after getting home with a wet dog.
I'm going to try a mat from Natsmats.com and see how they compare.