Photo # 1 - Next I position the insert backer boards in the positions I have marked. But before we cut the foam, we must take into consideration the width of the upholstery material which we will be wrapping around the edges of the insert and around the edge of the surrounding foam. This material takes up space. And we want the insert to fit within its opening without distorting the fabric or creating wrinkles or creases. By the same token I donâ€™t want huge gaps between the insert and the surround. So this part is a little bit of guess work.
Based on my practice panels, I am using a margin of about 1/8" all around the insert. And to create that margin I use a slightly blunted (read - used) Sharpie felt tip pen to trace around my backer boards. The Sharpie makes a line approximately 1/8" wide.
Photo # 2 - I position a straight edge along the OUTSIDE edge of my Sharpie line, clamp it down, and make the straight portions of all the required cuts using a razor knife.
Photo # 3 - I then very carefully freehand each curved corner with the razor knife. Go slow and make 2 or 3 passes. Donâ€™t try to cut the full depth in just one pass.