Its is recommend to use Top Down modeling techniques, to get all the basic reference (edges/point) and design it as new part from Assembly. This is for complex, hard to model around it directly using normal method.
First, crease a New part once the model inside assembly. This new part later will be save externally from the assembly.
Use 3D Sketch to convert the edges for your references. (green line)
This is the converted edges on a new part.
Next, make reference on how the inner structure (using plane )
Using Loft-Surface technique, create the patch of the lamp cover and continue afterward to other area.
Keep checking with assembly so that the Surface that you create fit correct to the lamp edges.