This is a method I developed a few years back for my day job. We needed watertight containers made from PETG as we were handling chemicals that would easily diffuse through – and break down all other commercially available thermoplastics. After some trials with sand I started using finely ground salt.
It perfectly immobilizes the parts, and gets only superficially embedded. Once the parts are baked, the salt will have superficially embedded itself into the part surface. Washing the part in warm water will dissolve all the salt, leaving behind a sparkly texture.