Predator guards are key, here is a link to the Maryland Wood Duck Initiative:
http://www.mwdi.net/MWDI/Predator_Guards.asp
Almost as critical as the predator guard itself is making sure that there is not easy access to the box from adjacent trees or vegetation
Would strongly encourage everyone installing and maintaining wood duck boxes to use predator guards. No sense creating the habitat only to feed the racoons and snakes
Don't know where you are located but you might have concerns about the box getting too hot if you used pvc. Check out the Maryland Wood Duck Initiative website; http://mwdi.net/mwdi/index.asp. Cliff Brown with MWDI is about a knowledgeable a person as there is when it comes to wood duck boxes...