Skipnchar has some good advice. Use some spacers. You can make your own with plastic cut up PVC pipe or whatever you find at the hardware store. Use your imagination. I'll add you can also use a nylon tie down strap to to hold down the battery box.
For my friend, I simply cut off his existing metal battery rack and welded on a custom sized one to his specific needs. Simple angle iron is your friend. You can find a welder to do that very simple and effective mod, if need be.
I'd do it for you for a beer and pizza, but you and I live at two different coast lines.
Silverado 2500HD Duramax DIESEL/4x4/CC/Z71:W
31' Travel Trailer
2 Honda EU2000i's (parallel)
ICOM 7000
Random wire, dipoles, verticals
Hunting/Fishing gear
Welding/Fabricator
Life endowment member NRA
:B :B