Ownpets Dog Cooling Mat, Large 39"x 24" Instant Cooling Pet Pad, Thickened Portable Pet Cool Pad for Dogs and Cats, Leakproof Water-Filled Summer Relief Mat for Crate, Floor, Car, Outdoor

This one is a little different from the norm. I tried it, and I thought it was quite cool. My dog wasn't into it, but there are pictures of other dogs on it, so I think it depends on your dog. It's a water-filled mattress. Reviews are mixed, but might be worth a shot. Link to Amazon.