This is a bit of a late response but .. my suggestion is to look at REI or some other outdoor
sporting goods shop and find a waterproof Lexan box. I am using one made by GSI. http://www.gsioutdoors.com/products/pdp/lexan_l_gear_box/storage/
Lexan is very hard, but not brittle and will provide a number of years of service. It can easily
be drilled with a wood bit. It stands up to UV exposure very well.
Beware of any plastic box whose composition you can't determine. (Hint much of the plastic these days is recyclable so you can normally determine what sort of plastic it is by looking at the codes). Stay away from Polypropylene (sometimes abbreviated PP, plastic recycle code #5) as it will quickly deterioriate with UV exposure. Many food storage containers have LDPE lids but the main part of the container is Polypro. If you use a polypro container expect to replace it
once or twice per year as it will crack and crumble.
One piece of advice ... I suggest you drill a small hole on the bottom side of the box to allow any accumulated moisture to drain out. It is virtually impossible to ensure that the box remains 100% moisture free so it is best to allow any condensation a way out. Also I suggest that any connection holes that you drill, you should also do so on the bottom.
I have used one of these GSI boxes to house a LDG Original QRP autotuner on the side fence in my back yard for a few years. I use the tuner to match a sloping 30m delta loop fed with open
wire line. If you would like to see some pictures of my installation drop me an email
at VE3WMB at RAC dot CA.