How to create a perfectly flat surface in Spaceclaim?
 

I have the geometry of an airplane cut in half, like this: https://i.imgur.com/qKJV7w1.png

To generate the fluid domain in Spaceclaim, I have to create a rectangle around the geometry at the plane of symmetry and then extrude it (correct me if I'm wrong).

The problem with that is, when I fill the remaining face of the geometry like this: https://i.imgur.com/wDK0CkN.png

it creates a not-flat surface, which absolutely ruins the fluid domain generation step.

https://i.imgur.com/j9Warsy.png

It is barely noticeable here, but you can see the surface bulges out just a little bit.

When I try to cut one surface from the other, it actually works, but it creates a hole in the fluid domain.

How do I generate an actually flat surface in this situation?