aka get hoarder in short.
Buy and/or equip the Hoarder mod, this makes enemies always drop materials, and the enemy we'll be hunting should drop it most of the time.

Any mission in this level should work, but i picked the 2nd mission save ironside for this.
Since you can quit mission and keep your materials anytime no need to bother with the mission itself.

Next up head left and take the door nearby, it will take you right next to our spot.

Once youve got all set up and youve got to this spot with 2.... spear dudes and a box, run back and forth the area to respawn them and kill them with every pass, you should get a fabric with every kill (they rarely drop wood too)

I ran and jumped on the pillar at end of both ways to go far enough for the enemies to respawn, sometimes they suddently stop respawning or only spawn on 1 side for me, im not sure what controls that. You can try exiting and re-entering the area if the enemies dont feel like respawning.

Lastly all you need to do to keep the fabrics you've gathered is abadon the mission before the time runs out and select to keep your items.
For the rest of the materials needed for it, killing the last boss should't be too hard once youve done it once. I use a minigun against him to collect money currently and he usually dies from a full clip from it.