When it comes to no-utensils #outdoors #cooking you can rely on the old faithful - meat wrapped in clay and put deep in the fire.
But there's another less known method - steam using moss. Probably not gonna work for a large hunk of meat, but wrapping fish in sphagnum moss and placing over medium coals would do the trick beautifully.

More info:
https://blog.raymears.com/2015/07/29/how-to-cook-outdoors-moss-steamed-trout/