Short of using proper dreads (which really make or break a build) your only other options are to either make some from thin foam mat, or (shudder) use backer rod. The biggest mistake people make is they use 3/4" backer rod, which is way too skinny. If you have to go the backer rod rout, do yourself a favor and order some 1" backer rod. The difference is huge, and it's way more accurate (if that's possible) than the anorexic 3/4" everyone uses. Search google, you'll find tons of suppliers for the 1". Better yet, save your money and but a proper set of dreads... you won't be sorry.
Brian