I was thinking some sort of mesh.
Not everything is a filter in Illustrator.
Starting with four lines:
- Blue = Waves
- Red = Frame
- Green = Spine
- Selecting the Waves > Menu Object > Blend > Make
- Menu Object > Blend > Blend options > Specified Steps = 20
- Selecting the Blend and the Spine > Menu Object > Blend > Replace Spine
- Menu Object > Blend > Expand
- Menu Object > Ungroup
- Selecting the new Waves and the Frame, get the Shape Builder Tool
and click where the shapes must be filled
- Fill them and delete the unused paths.