It might be a little small for your scale, but I like braided fishing line. It's stiffer than string and doesn't stretch much but it still bends easy and you can get it in dark colors. I've been using Stren, I think.
It's been a few years since I bought it but I think this is the stuff. Biggest is 80 lb test and .014" diameter.
http://stren.com/sonic-braid/
Looking forward to seeing your build on here. I've got a Bucyrus Erie 25B in progress, though progress has come to a halt since I moved and lost my garage to work in. Hoping to remedy that next summer.