Dont think too sublime when you think gravad laks. Its very old technique if memory serves me right salt from saltwater was used as a preservant[hole was dug in sand]
There is another version of it, called rakfisk, something utterly disgusting. My boss was doing it, left the whole arctic chars for a year in a bucket, outside. You can imagine the smell of that shite. Im not sure if "zyklon B" was more dangerous.
So basically find a cool spot in the nearby forrest, I would say 40-50 cm deep would do it, you can put the fish in alu foil or parchment paper but you can also set it just like that.
The heat from bonfire is radiant so the salmon doesnt need to be hanging directly over the fire.
Dont forget to gut the fish, take gills and eyes out
I think it would work with two sticks on the sides as a "pillars" but maybe easier is to dig a hole just long enough so the fish can be set over it, and throw coals/wood into the hole. So then when the coals are ready, and fish is on the "skewer" you just have to keep turning.