I’ve had good luck using sil-plate Foam Gaskets on the inside of waist belts. I use the 3 1/2″ wide/1/4″ thick foam that comes in rolls. It’s easy to sew on a nylon cover or to cover with adhesive backed insignia cloth.
Here’s a photo of a 30″ long piece next to the buckle and webbing to which it will be attached after covering.

