I like pine shavings best. about 2 inches deep, no more (a few eggs tend to get lost in deeper shavings, and not hatch).

Cedar chips work good to keep insects down, BUT cedar chips have also been know to cause respiratory distress in some birds and small mammals. Go w/ pine IMO.

Make sure the box is slightly tilted forward, to help the ducklings get out easier.

Predator guard - is a must.

Hiegth? Above the flood plane. Wood ducks usually nest in tree cavities above the flood plane. Found one such "used" cavity - 60 ft up. Best IMO, to place them where you can access them easily to clean/check, and be above the flood plane at the same time.

Just sayin....

T.