I agree that expanding foam can be useful for this, especially if you have varying sizes/ spaces to fill in addition to easily-cut foamboard shapes. I got one of these last December--
http://www.stonetooldepot.com/index.asp?PageAction=VIEWCATS&Category=328
-- if you're going to be doing this (expanding foam) in any quantity, it's far easier, more controllable, and less expensive than the hardware -store throwaway can-with-straw foam dispensers. You can also use part of a foam cartridge, close the gun (via a knob on the back) and set it aside for hours or maybe even days - and then come back and use the rest of the foam (unlike the throw-aways that clog themselves shut if set aside for more than a brief while). Just _don't_ get it on you- it's quite an adhesive (I've actually used it for that, also, in a pinch, and for non-critical uses).
You can get these other places, but the above place has a location near me, and their prices seem better than some other options I'd found (and they're good folkls)