I found a couple battery straps for sale on a few Hobby shop websites and decided to make my own.
Get two 3/4"-1" links from a chain as the buckle for your straps.
then I took 8-9" strips of strapping tape that were cut to fit inside of the chain links.
tape the strapping tape through the link a couple inches on one side, like you are making a belt. Tape the other piece of strapping tape going through the link the other direction, so the tape is facing itself and sticks together. So there are basically 4 pieces of the tape going through the link itself and reinforced for a couple inches and then the rest of the strap has 2 pieces of tape that are stuck together for added strength.
Now, put your batteries in the car and put the strap around the batteries and chassis, pull the end of the strap back up through the buckle and measure how long the strap really needs to be for your application and trim it at that point.
Take note of how long the pieces of velcro will need to be on the parts of the strap that overlap at the top of the battery, usually a couple inches at most.
Take the strap back off, and apply the 2" hook side of velcro to the end of the strap on one side. Leave about 1/2" of space and place the 2" fuzzy side of velcro to the same side of the strap.
I positioned the strap where I wanted it on the bottom of the chassis and then put a 2 1/2" strip of strapping tape on the inside of the chassis towards the strap at the bottom just to further secure the strap in place.
I've raced several times with many severe accidents and have not had my batteries come lose one time with these straps.
Here are links to some pics.
Battery Strap pic 1
Battery Strap pic 2
Battery Strap pic 3
Battery Strap pic 4
Battery Strap pic 5