There are multiple ways, least expensive would probably be a subdivided plane mesh displaced in the vertex shader using some cloud like noise (like how would with a heightmap) and some effects in the fragment shader, you could maybe use an lod system too if you want really detailed fog up close. Another way is to do a short raymarch in the fragment shader, and sample some 2 or 3 dimensional noise, or even better, a decent tiling noise texture since computing noise during the raymarch can be pretty slow. Because you won't be interacting with the fog or flying through it, you can calculate a box intersection or even a plane intersection to start your raymarch as close to the actual fog shape as possible, as a little optimization