I'm debating on making one myself. Something that is locked up by having the rear gate closed and locked. It would be nice to have my recovery gear secured. Plus the top would be a good place for the cooler. I have seen ammo cans and some plastic bins that you can get from Walmart. Works, but not quite as clean.
I'm thinking along the same line as Matt, building one myself. I'd like to mention a related thought though: Put very briefly, locks are only meant to keep honest people honest. If someone wants in, they'll find a way regardless of how "secure" your lock box is.
Personally, I'd consider investing the funds into a silent perimeter alarm system if I'm to keep expensive equipment in/on my Jeep (although I'm not there yet). These alarm systems are getting quite advanced. They're not cheap, but neither are your tools and equipment investments, not to mention possible upgrades.
Sorry I didn't provide a better answer, just tossing ideas around.
The only problem I can see with making your own might be can you weather proof it. I live in the Northwest where we get copious amount of rain a year. When you don't have a way to get air to dry the back out everything starts to rust. Plus I live on the coast so the salt air is a factor is present along with the wet. So it would have to be sealed totally for me. Might try a contractors tool lock box that you could secure from the bottom and lock from the top. Just a thought. Good luck!
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