Before having our camper, we tented a lot and all over the country. I picked up a plastic pickup truck tool box that slipped in between the bed rails and sat on the floor. Very weather resistant. I know I did not pay for it and it has been very good for camping supplies. If we were car camping we could get most of our gear into it. If you were a bit closer, I would tell you to stop by and pick it up.
As for sealing the wood. Personally, I would mix up a mixture of Boiled Linseed oil (must be the boiled type) and Turpentine 50:50 ratio or more turpentine. Brush it on until the wood will not absorb it any more. It soaks into the wood pores and seals it up really nicely. The only downside, it can take up to a month to fully dry, especially if it can soak in deep. Another option, which would be more costly, is to take epoxy resin and fiberglass and coat the box just like a modern wood canoe/kayak. Depending on the box size, might be $100 in materials. As for epoxy, I like the RAKA brand, it is a bit cheaper and very nice to work with. You would be surprised how far a 1 gallon will go, depending on your box size you should be able to get by with a quart of resin and a 1/2 quart of hardener, with a 3-4 oz glass.
Here is a picture of a box similar to my Rubbermaid box.