Sorry, I uploaded the wrong video, I’m changing the previous post. Refesh your browser page and watch the correct video and you will see I created the 6 surfaces using the Skin tool. I did not cut the STL file except for the two non-planar edges of the openings to make them planar as demonstrated in another video.

I ran into difficulty on the tail section and ran out of time to try again (and again), that was the video you just saw.  It might work out for you. If not, I suggest you make one cut to separate the STL facets into three pieces: Leading Edge, Upper Tail and Lower Tail. In that way, the Skin tool will easily cover those three facet bodies.

In the end, you can use those three surfaces to split a solid block and end up with a solid wing.  Good luck!