The draft shield is quite effective of keeping the temperature around the model thus reducing the up-dragging force caused by the middle layers' shrinkage. However the shield itself still shrinks which results in warping if the raft is not large enough. The key is to cancel the up-dragging force. Here is a box for example.
A thin vertical crack on the shield allows the middle layers shrinks more freely without affecting the lower layers. During printing you will notice the shield breaking, and that is how the up-dragging force is released.
A crack with sawtooth allows the shield breaks more easily, as I still found slight warping when the shield was trying to break.
During printing when the model is built higher, the horizontal crack of the shield is also growing, which leads to more heat leakage. We can overlap the shields to solve this problem.
With a model even higher, another shield with a cone or pyramid shape can be added to collect more heat from the bed. But I haven't tested this idea yet.
The shields that manually created cannot be recognized as a single wall object by Cura. You have to make it thicker than 0.4mm(nozzle diameter), e.g. 0.41mm, and Cura will proceed it into two walls, so the printing takes a bit longer.