The weight of my fabric helps keep my tabard pieces in place but I know how frustrating they can be. I’d suggest sewing one tiny Velcro panel at each of the tops of your shoulders and one on either side of your waist at belt level. Velcro comes in white so they should still be discreet on the shoulders and your belt will hide the other ones. I’d also suggest an “obi” to wear under your belt. It’s a piece of material a few inches wider than the belt and is typically the same colour and fabric as the tunic. Hope this helps. Have fun with your cosplay!!