I have a home built "Truck Vault" type bed storage unit that was fit to my Tacoma Double Cab. The drawers are approximately 5ft long and have Unican locks. The vault is extremely secure and has served me well. Here is a link to the craigslist classified. $500 for the unit.
Drawers are 7" high (inside measurement). I used MDO to keep moisture under control and there have been zero issues. The bottom runs on hard plastic runs(bottom of drift boat UHMF?) allowing for easy movement, but no so much at they slide out on a hill. Here is the updated link: