You can make custom size boxes through many fabricators like AAA, DRE, boatworks in Idaho, etc. I warn you it will be pricey. I special ordered both of my captain boxes to size through AAA, they missed the size by 1/4 of an inch (miscommunication) but I made it work. So when you order special size, be as specific as possible! Custom boxes will run you around $240-290 per box.
This is how I mount 4 (2 20mm rocket boxes, 2 30mm ammo cans and a jug of water) in my front bay:
I put the table on top and strap it down. This setup has been flipped in Westwater and swam upside down through Skull and Bowling alley without any problems.