I make seat belts in 1/72 scale out of good quality tissue paper, the kind used for wrapping clothing before boxing. I cut really thin strips - shoulder harnesses are a little less than a mm or 1/32 inch, seat belts about 1 mm or a bit more than 1/32 inch - with a new blade. The tissue paper is more flexible than ordinary paper and if it's good quality, the edges don't "shread". To apply, I put a little bit of white glue diluted with water on the strip, which makes it easy to position realistically and keeps it in place. If white isn't the appropriate color, I paint the straps after they are in place, which is easier than you might think, since the paint tends to wick into the tissue paper if you don't use too much white glue. I have a couple of sets of 1/72 PE buckles and other harness hardware but I haven't yet used them because they are so incredibly tiny. I simply dot the harnesses in the appropriate places with a little aluminum colored enamel.