Hello,
Few options I can suggest:
1. to use a CAD software to create your desired 3D geometry and export it as STL file; then you can import it to Lumerical from top menu:
2. You can use wavy layer from Object library as reference, update the script and make it to your desired geometry.
3. Use custom structure Custom structure object – Simulation Object – Ansys Optics
4. You can use equaltion and build from geometry.
Thank you.