[Christ]

I'd just hit up a construction site for some pieces of foam board and use them, if you can.

You might also have luck (if you don't mind getting dirty) finding a party or something where they're using foam cups (wedding, etc) and seeing if you can get them to keep the cups separate for recycling, or if you can even take the throwaways. If you crush them up, you can insert them in vacuum bags and make decent insulation panels that way, too.

Lastly, Sand (I know... ) might work well. If you can put your battery in a battery box, put a layer of sand about 1/2" or 1" thick under it, and around all sides of it, then some spray foam to hold the sand down, and foam in the lid of the box, as well.

The issues with sand are obvious... it's friggin heavy.