I'm not sure if you are talking about securing the battery to the bike or for protection while carrying them around. In case it's for transport, this is what I do:
In the last year, I've met two people who damaged their batteries by mishandling. It prompted me to get protective cases for transporting mine. This company offers a wide range of different types and sizes:
Our blow molded case series offers lightweight blow molded polyethylene cases (HDPE) that are virtually unbreakable, tolerate high and low temperatures, and resist chemicals.
www.casesbysource.com
My bike uses 48V 15ah rear rack mount batteries which cost around $700. I usually travel with at least 3 in my vehicle. With that kind of investment involved, I do this:
I first put the battery in a soft padded case:
Bag for electric bicycle battery. Quantity: 1pc Bag(the battery is not included).
www.ebay.com
I then put the padded case in one of these blow molded cases for physical protection:
I found a case that would also carry the charger
I sometimes carry two batteries in one case to save space:
The cases fit neatly in my panniers when I ride with a spare battery. They offer protection from a spill.
Also consider where and how you transport batteries in a vehicle. A sudden stop or minor accident could cause damage if not stowed properly.
Overkill? Maybe, but with that kind of investment, why take a chance.