from Don Casey's book
CANVASWORK & SAIL REPAIR
Used by permission of International Marine/McGraw-Hill Publishers, ISBN 0-07-013391-3
From the back edge of the foam, measure across the top, the front, and the bottom and back again to the back edge and add 1" to get one dimension of the main piece of fabric. Add 1" to the width of the foam for the other dimension. The double width of the cover piece may be confusing, so pay close attention to the orientation of patterned fabric.
Measure the side dimensions of the foam and add 1" to both to get the side boxing measurement. Two pieces are required. Use a compass to mark a perfect semicircle at one end and cut the boxing pieces to shape. Put alignment notches at the noon, 3 o'clock, and 6 o'clock positions.
Make a zipper assembly 1" longer and wider than the foam's back dimensions.
Fold the main piece in half and notch it at the fold on both sides. Unfold it and align one of these notches with the center (3 o'clock) notch of one of the boxing pieces. Sew from the notch to the back edge, running the machine by hand initially while you sew the curve of the boxing to the straight edge of the cover piece. Turn the assembly around and start again at the notch-overlapping a few stitches-to sew the other edge of the boxing to the cover.
Transfer the noon and 6 o'clock notches across to the opposite edge of the cover with a square. Install the second piece of boxing the same way as the first, taking care to get the alignment right.
Trim the back edges of the cover to match the length of the of the side boxing, then staple the zipper assembly into the opening of the cover and seam it in place. Remove the staples. Open the zipper and turn the finished cover rightside out.
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