Styro is a great idea! I can get thin sheets and layer them. Probably even better than a single thick layer.
I was also thinking of using a rescue blanket as either the inner or outermost layer. It'd be lightweight and add a bit of R-factor. Or even do a styro-blanket-styro laminate.
This is sounding easier and easier.
I can't use the fridge in the office, so I still gotta buy one, but I've seen em for under $60 on ebay/craig's. Even with a decent inverter, sheet aluminum and drawer sliders/latches, I'll be money ahead.