How to secure a third battery on rear rack

Hi,

I just finished an ebike 24V-36V conversion. I need to place a 3rd battery somewhere. I opted for buy a Schwin rear rack from Walmart for $17.
I have a battery sitting on it held with a giant oring and a bungee cord.
Since this will see a lot of vibration I want to do something better.
I see many pictures here with a battery on rear racks. How do you secure them?
Thanks,

Big rubber bungies.

I do use a very solid rear rack ... seat post only models are generally not good enough.

You have to tighten the bungee enough so it sounds like a bass guitar string. Then no vibration.

I have my battery in a plastic tool box. I used some flat headed bolts sticking out the bottom of a tool box, the bolts are covered with shrink tubing. They are placed so they stick down inside the rack perimeter, to keep the box from sliding around. Also makes the box self aligning.

Wide velcro ! Industrial Bungee.. crazy glue !!

The seat post racks : I like the bolt on types.. the quick release never seem to stay put for me.. and the cheap schwinn plastic decks slide off the post.. keep an eye on it !! ( better yet.. do as i did and drive a set screw into it !! )

On my wife's bike, I add two rear side baskets. http://www.bikepartsusa.com/cgi-bin/category.cgi?item=01-85554&type=store&category=basket-rear .I put 2 batteries on one side and the third with the controller on the other side. This places the batters down low near the center of the wheel, and greatly improves the center of gravity, on the other hand, the baskets do increase the width of the bike a lot, and it requires more room in the garage or in a bike rack. There still a lot of room in the baskets for other items as well