I'm using something similar to this.

Amazon.com: Battery Tie Down Straps,Boats Stainless Steel Cam Buckle Straps Cooler Tie Down Kit with Stainless Brackets and SS Screws Used for Fuel Tank, Cooler, YETI, RTIC, RV, Automotive and Marine, Fual Tank : Automotive

If you want some kind of cushion under the battery you can use the thin foam mesh type shelf liner material from walmart. I helps keep the battery from shifting around.