Someone just posted a pic where they ratchet-strapped their door handles together and I thought it was simple and secure, if a little inelegant. I've seen regular sliding deadbolts, like you'd see in a bathroom stall used, too. Between one of those options and some tinted (as-allowed) security film on the inside of the glass, if someone's trying to break in while you're in there, then they probably didn't know you were in there and don't want to interact with a person while there, so you can poke them with a pointy stick until they leave at worst.
If you're worried about it while you're not in it, then the same setup with maybe a hidden shackle lock on the back doors for long-term parking re-entry would be fair, coupled with an off-the-shelf alarm system.
Thanks! The Stow N' Go is an incredible invention in itself and worked better than I could have hoped for the way we built over it. We basically doubled the amount of storage by implementing the slide out drawer on top of it. The only reason I even know it existed was because I rented one last minute to assist with a friends move late last year and the second I saw it I knew it had some potential in a minivan build.
The hitch accessory is called a Rak Attach: https://www.1up-usa.com/product/rakattach/
There's another company who makes a similar one called the hitch swing as well: https://www.wilcooffroad.com/shop/hitchswing/
The thing you have to be aware of is the clearance height though. We have to actually install a hitch riser this week to lift up the rack a few inches because the arm of the swing accessory bottoms out when going over steep curbs if you don't take them at the right angle... this is more of a problem on my van than others since Minivans are more like Cars with their gate height than a cargo van or pickup truck, but there are easy solutions to that like a hitch riser which are under $100... I may also eventually swap out the rear shocks to a "load leveling shock" so that when we're weighed down we don't have too much sag in the back which was contributing to the bottoming out issue as well.
The work surfaces on the kitchen galley in the trunk and the slide out drawer are Bamboo.
The lock is a standard "Hasp Lock" that I got from Amazon: https://www.amazon.com/gp/product/B002DY7DRY/
Overall, I really lucked out with the guys who helped me on this. I posted a Gig on craigslist and got more replies than I expected and was fortunate that these individuals responded. Normally they work on much larger scale / higher budget projects but were in between jobs and thought it would be "fun" to do... only took 3 weeks to complete from start to finish. As far as "professional" conversions go, I got more than I could have hoped for and was beyond blown away by the level of craftsmanship they delivered. Nothing rattles, everything fits like a glove and the best part is that I can actually easily reverse the conversion back to the original setup if I really wanted to.. we didn't drill a single screw or hole into the van whatsoever. Can easily put it all back the way it was!
If for locks you mean make them more secure, I've heard good things about these
Master Lock M736XKAD Magnum Heavy Duty Solid Steel Hidden Shackle Padlock with Key, High Security Hasp https://www.amazon.com/dp/B002DY7DRY/ref=cm_sw_r_cp_api_glt_i_K6KHQT5ZRWS0WZ6AQSE4?_encoding=UTF8&psc=1