spare battery

I want to keep a spare battery with me since I have two skis. What ideas do you have for safe and secure storage? I don't want leave it loose in the trunk or otherwise in its cardboard box. Is there an option that could secure it on my trailer or secure it in the cargo of my SUV?

I have a 2005 Sportster Le DI with the ROTAX 947, 2-stroke direct injection. I bought the boat used, and the previous owner had an aftermarket stereo installed on the aft side. The stereo is fused and wired directly to the starting battery's posts. I want to put in an on/off switch so I...