Hard Ballast containters? Or move batteries??

I am looking to add more ballast to my engine compartments. I only have 200 lbs in each compartment. I want more.
I normally run 750 in the ski locker, and another 750 lb sack on top of the walk through. I am looking to upgrade to 450 or more in each engine compartment and add 350 to the bow so I can eliminate the second big sack in the walkthrough.
Problem is - I have two batteries in the right engine compartment and can't get a larger sack. Has anyone here ever used hard containers to hold ballast? Suggestions?
I believe this will be easier than moving the batteries - unless you have done that?

thanks for the help!