I've found that bolting a big box of anything to your work vehicle will come back to haunt you one day, or maybe it will every day.
If it's not part of a well thought out system, it's doomed to hinder you more than it helps you. That's why "bolting in" anything other than one of the numerous commercially made van storage systems, or your own analog, is a no-go IMO.
If they want it bad enough, or your your property is that valuable, crooks will simply steal the entire van and figure any locked box later. It's not like all vans are notoriously easy to steal or anything. Yet, you will still have to trip over and work around your bolted in box, every day.
...
Provided a properly kitted van with a front partition and "vanlocks" or similar, a straight sided metal box with a hasp and padlock for specialized, dangerous, and/or expensive items, like Knaack's "welders box" is about as far as I allow for.
That way, if the situation arises that an item needs to be left on site, locked in a guarded building for example, we can still assert our control over the item within the transportable locked box, the bailee (the guarded building) merely provides a guarantee the box will not be molested.
Simple and efficient for everyone, logistically and legally.
The other half of the system is to maintain "inland marine insurance" that specifically details all of the expensive stuff, and provides for a physical check delivered to the site of the loss within 24 hours, and now some fancy digital claim methods.
That way, we just cover the obvious basics and go about our own business, and let the insurance do theirs. It's really not expensive, just have to show written business policies and cover the basics like a partition and vanlocks.