Not that this is an option any more with a lot of modern cars. But I have a friend that has an International Harvester, Travel All (sp?) it’s big and has lots of room. He had a few tool boxes mounted under the hood on angle iron frames with straps to hold them down, he stored all kinds of things in them. He also cut a square hole in the floor in the back; he made it just a bit smaller then the frame and very long front to back. He made a metal box that extended down below the floor to store things. He put 2 doors on hinge’s, and then cut a carpet to fit the back area. This was a big box, lots of room! The back storage box and the ones under the hood were 100% invisible to anyone that looked at the truck. I’m sure it could be done with modern trucks (you may need to relocate some wiring) maybe do it with some cars. But mostly with larger trucks.

If you are willing to work at it you should be able to come up with something secure, and maybe even hidden. .