One big advantage to using the expanding foam inside a plastic bag while cores are in place under the cowl, is that any objects/obstacles are clearly imprinted into the cured foam block/ball. When you remove the foam and begin sanding to shape, you can easily see where object interference will occur and simply sand enough off those areas to ensure proper clearance.

        This works well to make a proper fitting duct the first time. The disadvantage is that sometimes the foam will conform so tightly to obstacles, that it takes a measure of gentle persuasion to remove the foam block.

S. Todd Bartrim
Turbo 13B
RX-9endurance
C-FSTB
http://www3.telus.net/haywire/RV-9/C-FSTB.htm

        "Imagination is more important than knowledge"
                                        -Albert Einstein