I second the SD box. We finally got one this year and the price is worth the peace of mind. Afterall, the bank has more theft and fire protection than I could ever hope to afford.

If, however, you still want to do the outdoor storage route then I would suggest surplus ammo cans. I have two geocaches stored in them. One is hidden in a stone wall and the other is exposed in a open pit (in the middle of no where!). Both have been out for about three years now with no sign of leakage.

Chris