[JRTJH]

I would not use self tapping screws with points toward the batteries inside the box. Doing so is a good way to have "cargo shift" slide the battery into those sharp points. I'd suggest removing the batteries, drilling appropriately sized holes, mounting the shunt (or the plastic box that holds the shunt) to the outside of the battery box with bolt heads INSIDE the battery box. That way, there's less chance of damaging your battery/batteries.