I think the area for these could be streamlined. D3x5cm by D3x5cm for asteroid and mine fields seems very clunky, it works, but I think it could be easier. For these, I think you should place 2D6-3 (min 1) 5x5cm squares of mine or asteroid fields.
D6x5cm by D6x2cm for gas/dust clouds is less clunky. Again we could break this down into a different system though. Take the base of a rectangle 2 cm by 5 cm in area. And we could do this two ways, you could place down either 6d6 of those bases per cloud, or d6 times d6 bases per each cloud.
Naturally one could use either square bases for these, or my preferred method, cereal boxes.