I am trying close of this surface with a flat plane. The issue is that the ends are incomplete making it impossible to use a single tool.
I believe I am in need of a compound of tools. Thanks for the help.
Here is the blend file (google drive) of the kind of surfaces I am working with, and here is a video (youtube) showcasing the kind of result I am after. As it can be seen in the video I am currently merging the sides and then using fill (ALT+F) to fill the plane, after this I fill the sides of the object.