I use a leftover piece of tile cement board just to keep under my chimney starter to keep from scorching my driveway when I use it. I've not tried it on wood decking before, but I'd think it would suffice. The stuff comes in a 3'x5' sheet, so you could cut it in half and double it up too, just to be safe.
Mine's lasted for 4 years now through rain and all and is still holding up OK.
I don't disagree with the metal oil drip pan idea either, though as suggested by otterpop. It'll definitely last longer.
If I recall, you're in a rental home, right? You roommate with "The Drunk"? I think I'd use one of these simple solutions instead of tiling somehing up that would be cracked when trying to move around.