Portable car power supply?

I'm working on a project. I'm a snowboarder, and I love my xm radio. I'm hoping to wire my vehicle only radio up to a 12 V battery so that I can bring it with me while snowboarding. I have everything figured out except for the battery part. The only 12V batteries I can find are either too weak and won't last long, or far too bulky to be snowboarding with. I'll carry a backpack, but I don't want to be lugging a car battery around while boarding. Any suggestions?