There's alotta ways you can put convincing belts and harnesses into a model; arguably the easiest is to buy some brass belts and throw 'em in.
But I've settled on two methods to scratchbuild them, one easy, one slightly less so; but both can be tedious and somewhat time consuming. The results are worth it, however.
You can use metal foil, aluminum, lead, whatever. I have a huge stockpile of foil wine bottle seals, and this stuff is ideal. Here's some shots of belts I put into a Mustang, sorry for the bad photography!
I used the foil for the belts and harnesses themselves, and for the buckles I used flattened .010 solder wire. It actually looks way better with my own eyes than in these photos. (even if there were no blurriness!) The foil bit is a bit superior in some ways to the next method, but not as easy...
These are some shots of a cockpit for a P-47. The belts, harnesses and buckles are all made out of masking tape. I shoulda used a different type of tape, this stuff has too much of a texture for the scale, but it looks pretty groovy, nonetheless...
Again, sorry for the bad photos...
Fade to Black...