Hi... they definitely do make one - I recently bought an IS 300 SC and it came with a Lexus branded rubber mat in the boot - it's the exact shape of the boot area with walls that come up about 1.5 inches all round - it's pretty thick but a dog with sharp claws would probably ruin it after a while.
Afraid I can't see any part numbers etc on it but just wanted to let you know they do exist!
Oh, and they're definitely waterproof as I 'tested' mine by emptying a big flask of coffee over it at the weekend! :)