I had to think how I constructed the boomerang shaped parts. After some puzzling I noticed there is a very easy way of constructing them:
(see images I below (60 and 45 degrees, that means 6 parts or 8 part in the youtube movie above we used 10 parts)
1. Draw two grey lines which define the angle in which the boomerang part moves.
2. Draw a red centerline between the two grey lines.
(notice that the joints always stay on these three lines)
3. Draw the green-line perpendicular to the red centerline.
4. Draw the bleu line from the intersection of the red and green line perpendicular to one of the grey lines
5. Mirror the green line into the bleu line
6. Draw the black lines as you see in the drawings, the ends of the lines are the joints.
Hope you manage to construct your part yourself. And let me know how it works out, or if you have any questions.