I'd suggest considering looking through these pre-built boxes and mounting/modifying to fit your need. The doors are usually already sealed to keep moisture & road grime out..
Tool Boxes | Iowa80.com <-- pages before & after 6 -->
Also I believe the Iowa80 truck stop store is a weee bit pricey. So shop around places like truck dealerships, RV dealerships, trailer dealerships <-- they hang boxes off flatbeds on nearly every trailer. **Editing original post. Scroll down to the post "Under Body Boxes" They have other ideas for sources like Tractor supply & such.*** Depending on where mounted I would not go with two doors that swing out. Road vibrations tend to cause them to wear and I've had doors like that blow right off at highway speeds. Ooops! Umm boss you know that stuff that WAS in the ditch side box? Yea not a fun phone call. As an owner I'd be like demmit! Now I gotta buy all that crap over.
I'm def going to be in the market to be considering my options for carrying things when I get a van. One option I'm thinking about is taking one of the stock bumpers off and welding my own (with a welder friends help. I want to learn welding and this would be a good project.) Or maybe leave the bumper, run a couple rails from the front frame and build a platform & box to carry the generator & other accessories.
Lastly; I've had both espar & webasco heaters in trucks I operated. Those things will roast you out!! Even in below zero weather. Side note. It only takes about a half hour (steady run) to heat a can of soup up in front of the hot air vent.
Just sayin