My point is that it does not matter if the set is private or not.
If you want to train your model you'd need more data than the private set anyway.
So you have to build a very large training set on your own, using the same kind of puzzles.
Yes you can build your dataset of n puzzles but it was still really hard for any system to achieve any scores, it even beats specialized one for this just one task and this puzzles shouldn't really be possible just to be memorized by the amount of variations that can be created.
If you want to train your model you'd need more data than the private set anyway. So you have to build a very large training set on your own, using the same kind of puzzles.
It is not that hard, really, just tedious.