I cut one of my K2 batteries open, after one of the cells in it failed. There's not a lot of space available in there for foam, or drill bits.

-nick one of the cells with a drill and it's going to be bad.
-Equal foam filling is unlikely
-Good possibility of the foam compressing the cells or expanding the case.
-The foam is going to interfere with heat dissipation in a big way, its a pretty good insulator.
-When chasing vibration issues, sometimes tightening up the system makes things worse not better.
-LiFePO batteries don't supply their own oxidizer in a fire, No one ever said they wouldn't catch on fire. Just if they do, they can be put out by depriving them of oxygen. Unlike Lithium ion chemistry batteries which supply their own oxidizer if they catch on fire. So the LiFePO batteries are considered to be a safer battery chemistry, but not a completely safe battery.. A plastic box full of lead plates submerged in sulfuric acid isn't completely safe either.

A LiFePO battery(s) with an internal BMS board, with a fuse right at the battery is the way to go, but treat it with the respect that that much energy stored in a small package deserves.

SF