Petroni.
I use a terracotta pizza serving plate as a heat sink in my 14.5" WSM. Works perfect. The water pan is now collecting dust and cobwebs.
The link below will show you in a bit more detail. (I just foiled the plate for easy clean up).
http://tvwbb.com/showthread.php?61914-14-5-Water-Pan-Replacement
Hope this helps.