To me a green burial can range from this:
http://www.acfnewsource.org/environment/green_burial.html
...to putting someone out in the back 40 and let nature and the elements take care of it. Of course, I mean no disrespect, I'm one of those who believe it's a bizarre practice to bury someone in a metal coffin than into a cement vault six feet under....and I think my feelings reflect historical sentiment.
Like MM said, it's a matter of respect, treat his ashes as you would his live person and I'm sure a solution will come to you.
ps, I don't have a will, but I have let my parents/family know my wishes. However, I do plan on filling out one of those simple living will statements.