I got a hard plastic box from James Lucky in the fall. It cost ALOT more than the army boxes that others have posted about, but it seems to work. I wanted it to carry a camera, a full size tire pump and some munchies... it works great.

 

I keep getting these plastic milk crates when I buy four gallons of water and after I ran out of ideas of how to use them I decided to make a box for my grizz.
I took three of them and cut one side on two of them and two sides of one of them (Center). I then tie wrapped them together and attached them to my rear rack. I use tie downs to secure the items inside. It works great. I can carry chains, saws and wedges when I am cutting wood, or I can store the 5 gallon plastic gas cans I need when I go further than 40 miles on this thirsty animal with room enough to carry all the other necessities.
Hey I'm not cheap just frugal......

 

All your suggestions are better than what I did. I bought two rack bags from Cabela's, about 40x12x8 (two different sizes for two size quads)--the material is tough & waterproof, all sides are padded, and they hold down well with a bunch of velcro straps, but the movable dividers are a poor design (velcro-ed) and heavier stuff breaks them free. Maybe I'll figure the most usable arrangement and fasten them permanently somehow.
BTW Cabela's also had a very nice black aluminum diamond-plate box but it was way too expensive-close to $200 I think.

 

Thank you all for you input and I am sure others have taken some of your ideas and have used them. Here are some pictures of what my storage box looks like. I have been thinking about moving it to the front rack. http://y42.photos.yahoo.com/ninman_ny