You can use “Erased Alpha” meshes inside the viewport to effectively mask the ViewportFrame:
I just erased the Vertex Alpha of the mesh, which causes it to erase the ViewportFrame where it is. I did it in Blender, and exported it as a FBX object.
Here is the place, you should just be able to clone that StormMask Mesh and keep using your same exact setup (make the vieport frame background visible, and just replace your sphere with the StormMask, that’s it):
ViewportFrame_Masking_FortniteStorm.rbxl (148.1 KB)
If for some reason you can’t just clone it, I also uploaded the sphere with Erased Alpha as a developer item: Erased Alpha Sphere - Creator Store