Have you looked into dense pack cellulose? It offers many of the benefits of the spray foam (Like Icenene) at a more reasonable rate. The problem with trying to cut foam to fit (other than labor intensive) is that to get it in, you need it smaller than the cavity, allowing convection around it. that means caulking or foaming all joints to stop. That's a lot of work that becomes un-needed w/ the cellulose.