I finally got around to trying something I'd read about in one of the magazines years ago. I picked up a couple old watches at a garage sale for $1. I busted them open and ripped out all the tiny parts, gears, springs and sprockets and even some of the small plastic circuit boards. I used a rail nippers to cut up some of the larger pieces (especially those that were easily identified as a watch part or a circuit board) and dropped them into a cup with some Blacken-it. After sitting over nite, I rinsed them with water and sorted them into 2 piles: big and small pieces. These parts make great filler items for those empty boxes that Brett includes in all SW kits.
I lined the bottom of the boxes with the larger parts and used the tiny pieces on top. In this extreme close up, you can tell the gray piece on the left is plastic and some of the white gears in the right-hand box also look a bit like plastic. However, at a normal viewing angle, those clues disappear and you get a collection of non-descript "stuff" with subtle color differences:
Sometimes just repositioning the boxes makes the the plastic-look fade as well. (Looks like the glue isn't all the way dry yet from this angle.)