At the recommendation of Larry Renger, at the 95 Nats, I started using Kevlar thread (.010) for 1/2 A lines. Actually, up to small and light .15 powered planes. It WILL take the 15 lb pull test! ...and it WON'T stretch!
The product I use is the Kevlar Thread, .010 x 1000 ft(!), 22lb test....last roll was $7.50 from Model Research Labs.com, in Mission Viejo, CA.
You can make up any length you want, tie it to the clips with a couple of half hitches or a square knot, and go fly. You can also just make a loop in the end and knot that. If the lines are too long, just cut them down. If you want them longer, just tie in a new section (I have NOT done this, and am NOT recommending it, but with this stuff, it's just like working with very small rope!)
Dave Gardner
Renton, WA