Montbell UL down jacket alternative or modification

I'd just take some 3M adhesive backed velcro and put it on the sides to tighten it up when you need to. You can also use non adhesive velcro and some seamgrip, probably would last longer. Put your hands on your hips, place one part of the velcro on the finger side, one on the thumb side, and when you want to tighten it, just pinch the jacket together on the sides.

You could also buy some elastic and seamgrip a 5" strip slightly stretched out onto each side along the bottom and clamp it down while it dries. That would probably work.

I like the elastic idea. I'm not a fan of velcro with these material.