Don’t know if this will work in the USA but…
The plastic covers that dry cleaning comes back in are a thin plastic that can make very good tarpaulins to cover anything. The plastic will both “sag” when pushed into hollows and take tension lines without splitting. This means that you can get it to look very like a real tarpaulin.
Easiest example is to take a few small blocks of wood and stack them like crates. Then cut your plastic to size and lay it over them. Where you want it pulled taught you pull it taught, glue the edge down (ordinary clear glue) and clamp/weight it in place until it sets. Where you want sagging you just puhe plastic into the space with a finger to slightly stretch it. Getting glue in between the blocks and the plastic can be a bit fiddly but works with practice. (You usually have masses of the stuff from one cleaning job). Once it’s set/dried paint it matt black or dull green as you prefer. REMEMBER parts will always be unsupported … so don’t poke through it unless you want to model a hole/rip.
You can even add clear varnio make puddles in hollows once it’s dried again.
If you cut the plastic to full size sheet sizes you can add interest by covering your pile or load with a number of sheets.
The only thing I don’t know is whether your dry cleaning comes back in the same plastic ours does… Guess there could be an export market for the stuff?
Have fun 
