Maybe try some 'pannier' type racks where it will also be better by distributing the weight each side more evenly.
This way you can keep 2 on each side, there is not near as much force applied to the rack as if they were sitting on top of one.
Then you can use the top of the wheel for papers or whatever. It will handle way better too
There's a post from Leslie if you do some searches of him using old tyre tubes to support the batteries mid frame, this could also be an option?