Cut two pieces of durable fabric to the size of your matrix plus an inch in each direction for seam allowance. With right sides together, stitch lines as shown using a sewing machine (or by hand if you must), then chop off the corners so everything will sit nice when you turn it right side out:
Iron the rectangle if it isn't sitting flat, then top stitch as shown below to make battery pouches:
You can optionally add D rings or clasps at this step to make attachment points for your light. Stitch small bits of fabric around them and include them in your topstitched seam, plus a little X for added support.
Use Velcro tape at the openings of the battery compartments.
When you're finished, the fabric backing will appear as below:
Sew your circuit elements onto the fabric backing with tack stitches-- that is, sew and tie off the thread in discrete junctions. That way if one stitch gets broken, the whole circuit doesn't fall off of the backing!
