Scale Soaring UK

This is my preferred method; easy to draw on the PC, and as you can see, expensive it is not, even at today's balsa prices!
Sheet the bottom, lay wing on jig supports and sheet top...

The PSSA Mass build Sabre used a similar method to include the dihedral and washout into the wing build:

It's worth remembering that a wing is not torsionally rigid until all three elements of the D-section are in place...(web plates, lower sheeting, top sheeting) Thus, when adding the lower sheeting, for instance, it really doesn't matter if the wing is flat or not. It's not until it's jigged up and the top sheeting added that the wing becomes fully rigid...