Watch yourself on that one. It has to be a foam that will not allow water to penetrate the foam. That's where the proper stuff comes in. It has been used to great effect by Sport Compact Car on their 300zx.
http://www.sportcompactcarweb.com/projectcars/0006scc_proj300zx/And also by a fellow with a really nice Z31 did some work with it.
http://z31.com/articles/5/Both are Nissan engineers. Me, I'd spend the cash on the right stuff. It isn't that much more money, really.
