I agree that Fusion 360 isn't going to be easier than Tinkercad, which is probably the easiest program for 3D modeling, but also the least powerful.

I second JustMatt's suggestion that it's a good program to learn, however. The amount of tutorial information and videos could be overwhelming, but the skills you learn will take you a long way in 3D modeling.

If you're an OpenSCAD user, you should be able to build your model if you have the dimensions of the phone. For your current objective, it may be easier than learning F360 to accomplish that one task in the short term.

For long term, F360 is a good direction.

You'll need to supply more than "a dimension" of the phone. Length, width, height, protrusions, desired locations of openings would be valuable.