Try the procedure you used to create the mask, but this time, use Shell Extrude. When you get the mask out to the thickness you want it, then click on the Body mode to select the entire mask, then right click and click on separate. This will work better than using extract.

Also, if you extrude the outside instead of the inside of the mask, it might work better too. Those lines are caused by Wings pulling some of the vertices with it when you extrude on an extraction.

If I had a nickle for ever time a woman told me to get lost, I could buy Manhattan.