There are a number of older designs which use a single central spar, either a horribly warped piece of oak, or a less warped (maybe) 1/2 sq  balsa.  Examples are the Zilches, the Mackey profile, many of Larry Scarenzi's (sp?) designs, etc. I have dealt with this in two ways.  For the hardwood spar, I make a laminated spar with basswood top and bottom and balsa in between.  One could put a strip or two of carbon in if desired.  Make a laminated spar straight and it will stay straight.  For the 1/2 square spars I have laminated two pieces of triangle stock. I also move the spar down in the wing a little so the leadouts clear and adjustable leadouts can be used.