I think it's a generally known matter among letterboxers that for a good lifespan for your letterbox, you need to think hard about the container you're going to use.
My oldest letterbox in the wild, and luckily my first letterbox, is still out there and is doing well - but it's only been out for a little over two years now. After all the rain we've gotten today, I wonder at how it is doing; as I recall, this little guy is not in a Lock & Lock but some sort of knock off.
The only thing I buy right now is Lock & Lock for my boxes, and I'm sitting beside a huge mess of them for the upcoming Box ON plant-a-thon that is coming in my near future. They're water and air tight, and other than one of my boxes, the flaps stay attached where they belong. So pretty... so expensive. I just bought a couple packs of three that WalMart is selling for back to school lunches, which works out each box to be about $2, generally the lowest I can find.
But I do have fond memories of pairs of perfect-sized L&Ls available at WalMart on sale at the perfect price. I was silly, of course, and didn't buy any - thinking that they would be available generally year-round. They are not. Lesson learned: when you see them on sale, it is time to Lock&Lock up!
Hope everyone's letterboxes are staying nice and dry!