Options for filler wires

I was wondering if something like nylon weed-trimmer line can be used in place of expensive wire. The stuff comes in different diameters. It's a good idea to have a few extra conductors for future additions to the harness but if all we need to do is fill space, it seems like a waste to use good wire.

The weed trimmer line that I've used has not been very flexible, so you would be adding stiffness into the loom, which is the opposite of why you do a concentric loom. It is also likely to have a lower heat rating and may melt when you are recovering the heatshrink. I believe that you can also buy Tefzel and similar filler tube, this is a non conductor tube of insulation material that is the same diameter as the conductor filled wire.