If it were my project, I would use some garment weight leather, contact glue or ordinary wood glue, cut the leather to fit plus some overlap, apply glue to the leather/copper, wrap it 'til it overlaps, use a sharp blade and cut through the overlap and the base leather, peel away the excess overlap leather. The joint you have remaining will be the thickness of the knife cut. If you want to stitch it up, use a glover's needle and very fine thread, and an autopsy (baseball) stitch to close the seam. Once sewn, burnish the seam gently with a smooth hard item like a small glass bottle, and the stitching will virtually disappear.
But prior to covering the flask, drill a 3/8" hole through the copper. That is an escape port for the powder gasses should the flask be ignited when full of powder. Otherwise, you have a pipe bomb that will kill you certain, and those close to you. The leather will cover the hole, and the glue on the inside will seal it.