Bobbinalong

I think I've got this. Cut your template for the triangle in half vertically and add 1/4 seam allowance to the cut edge (to make a seam later). Place the half triangles to each end of your rectangular paper/muslin with the cut edges at the ends. Sew strings between the triangles, all light or all med/dark. I guess the second half triangle is sewn on last?

When you join the blocks, you are joining the half triangles, hence the seam allowance. Just remember to alternate the light and med/dark blocks to get the effect.

Thanks maviskw, for a good idea.