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Hi, it depends on the amount of Wonderflex used and what is used underneath, but the plastic itself would probably weigh 1-2 pounds.
To finish the back, you take a few small stitches and/or make a half-knot by making a stitch, then putting the needle through the loop before tightening. Do this a couple of times for security. This is the same technique used to finish any hand-stitching.