BABY PLAYPEN QUESTION - HELP PARENTS!

OK, look, I won't lie, I actually have a playpen for one of my puppies - but its being used to protect her from one of the other dogs that's old and doesn't like puppies until shes older and no longer bothers the older dog. It allows her to still roam around free and all that, but I have one major issue with it...

She is, remember, a puppy... She has peed in this playpen now a couple of times and I got the mat out and cleaned it very well with fantastic first and then with soap and hot water. When I place it back in the playpen it smells great for a while, but then later on in the day I can smell something nasty; turns out, its the playpen.

No matter what I do, I can't get the smell out of it; I've even left it outside for a full day and night to try and allow it to air-out. Do any parents out there have any suggestions for me on what to do with this playpen mat? I've looked online to just buy one alone, but their not sold separately from the playpens and I really think its ridiculous to have to buy a whole new playpen just for a new mat.

Anyways, thanks for the advice. (And please no bashing, trust me, the playpen has saved her life and allowed me to keep the two in the same room/house ever since I got it, so its not stupid.) :P

Its a plastic mat with I believe thin wooden slats or something inside of it. It was very cheap since it was just for a dog...