I have gained quite a few master builders since finishing all the puzzles, just by making them fix huts. I was in a panic earlier in the game because I thought I wouldn't get enough to open the box, but they seem pretty easy to get now.

I make sure that the first skill they get is farming since they seem to move faster when they are younger, then either research or building and I had a 73 year old woman gain master builder so it can be done.

Keep at it, the dedication will pay off

NN