Hey I use to own a F-350 dually with a 5th wheel hitch inside it so I could mount my regular bed box like I wanted too. I ended up getting a smokin deal on a side bed box from Tractor Supply. Those are only about 5' long in length 24" deep and 20 " wide on the top half to mount over the bed side then about 15" wide on the bottom half over the wheel well. I didn't mount it on the side because of the hitch, instead I mounted up in the front of the bed cross ways it was a little short so I just used some 2" aluminum angle to mount it on the inside from one bed side to the other. They're not as big as the regular bed box, it even held my Hi-Lift jack no problem and all the tools I needed. I keep my extra tools in the 5th Wheel storage area.

Or if you want to really go cheap go to a Army surplus store and check out some of the different ammo cans they have. They have a tight waterproof seal and you can mount small U-bolt through it to put a lock on it under the big latch. A couple of those should mount no problem inside front of the radiator support. I know this because that is what I did inside my CJ-7's bed. Heck I remember a place in NC back in the 90's called Old Sarge that even made speaker boxes for Jeeps out of 5.56 ammo cans.