I would not attempt heating armor without a lot of practice on other plastic, you can go from pliable to molten mess in the blink of an eye, you should be fine with approval, your belt will help to hold in and also cover some of the area.
Like your picture but glue between the elastic and ABS strip, try to use a soldering iron to make the holes in the elastic as it leaves a nice edge around the post and prevents pulling, OR if no soldering iron heat a screwdriver or similar tool on the stove, it will pay in the long run.
Also if you make the distance between the holes 5mm shorter than is needed it will give a bit of tension once worm.