Kevin Matney's Kits show and I use a 0.021" x 1/2" x 36" piece of carbon fiber laminate as the top spar.
1) Use a razor saw to cut a slot for the spar.
2) Mix about 0.4 ounces of finishing epoxy (Skin It or your favorite)
3) Dab the epoxy into the groove using a disposable brush.
4) Push the spar in the groove and adjust for proper fit.
http://www.cstsales.com/carbon_Laminates_36.html