We're moving some of the lighter / bulkier stuff in our xterra's emergency kit to the roof and needed a weatherproof box that didn't stick up over the top of the rack (garage door clearance). Got this and love it, well made and reasonably priced, under $20.00 on amazon.
http://mtmcase-gard.com/mtm-ammo-crate.php
Aside from that box they have some awesome storage items that I'm thinking of for my Jeep. They have trays that multiple ammo cans fit in, either the standard metal ones or some really nice plastic ones that look really versitile and perfect for camping gear, tools etc...
http://mtmcase-gard.com/ammo-cans.php