When I was an Art Department Assistant, I was asked to shop for Gak. The Designer wanted me to bring back samples of items that could be used for Set Design. Most of the items I brought back remained on the Gak shelf, interesting, yet useless. The ones that he used had 3 things in common.
1. They were big. Small stuff is for Props. You can't cover large areas without alot of work, nor does the camera pick up the tiny details in the background.
2. They were cheap and readily available. The construction could go and buy 20 the next day, or within 48 hrs. He loved anything plastic!
3. They were perforated. Lighting is one f the greatest challenges with SpaceShip type sets. Bonus if you can light through the walls.
That being said, seek out your local industrial drainage supplier type yard. It's a gold mine for Gak.