Is this what you are talking about?

http://www.homedepot.com/prel80/HDU...&search_text="tough%20box"&DRC=4

It might be the same as this:

http://www.contico.com/automotive-foam-wb.htm

Contico also seems to make other structural foam thingies:

http://www.contico.com/automotive-sub-foam.htm

Thanks for the tip - I will be looking around.

Jonathan