Any fabric store or notions rack will have 1/2" elastic. Make closures with duck-billed velcro (two pieces of hook with hooks facing in, joined at one end, mating to a 'pad' of loop velcro consisting of 2 pieces of loop joined at both ends and sides. The hook pieces no longer than 1 to 1.5 inches long, the hook maybe 3/8 inches shorter.)
You can make the elastic length adjustable with a simple friction adjuster made of a 1.2 x 1.2 inch piece of milk jug plastic with 2 parallel slits or 3 large pin holes (my preference).Thread the elastic through the holes (pull it through with stong thread) and pull out a loop as long as you need to shorten the strap.