I removed the rear seat given that it is cruel and unusual punishment to have someone belted in that seat while off road. I put in eye bolts where the seat and seatbelt hardware used to be. For now I have ratcheting tie down cables to secure gear in the back. However, I was thinking about maybe putting something in the back seat area sort of like a permanent tool box like you see in the bed of pickups. Any suggestions on what works well for storing things in a tin top?

I don't use a "Cargo Box"
I had removed my rear seat along time ago, installed a "Bunch" of 1/4"  "D Rings" (Some using the rear seat latch bolt holes)
and use 1 inch ratchet straps & motorcycle "Cargo Nets" to hold all my gear from flying around on the trails.
My fear was I would survive the roll over, but get WACKED by my hydraulic jack, tool box, mini air compressor, cooler, etc.
Also it has silenced all those RATTLES from that stuff sliding around back there TO HELP;
                What was "THAT NOISE?!?!?"..... Oh it must be something loose in the back there,........
                                    500ft later that broke spring leaf lets go right into your tire sidewall !!!
Now I know that doesn't keep Thieves at bay, but until a year ago, ("Dipstick" 2/2001 - 5/27/2010) my 85# Black Lab would lay down on top of the dust cover over this gear when we went out.

My Idea for you would be a Army Surplus Metal case that I think they used for Recoil-less Rocket rounds. it is the size of a large suitcase and opens like one. The nice thing is that they are water tight.

Interesting idea on the rocket box. I do recall seeing a few cases at the Army surplus store some time ago. One of those just might do the trick. I'll swing by this week to see what they have. Thanks for the idea!