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How to use a set of lines from a derived part to create a new shape?

Context: https://cad.onshape.com/documents/efbbafb6b05d800790768197/w/c4dfe0ff6af112e209c97839/e/ef4e701aef64290f85b45bfe?renderMode=0&uiState=6882572da27d1511c889e3bd
I am building a box for an intercom. I would like to use the edge of the intercom (the blue line below) to create a new shape (in the current part studio, the intercom is a derived part), slightly offset on the plane of the top of teh intercom, from the intercom (the red line, it is just a fragment for clarity) that I will later offset and continue with the enclosure.
My problem: I do not know how to do that. I tried to select the edge, and then "offset" but the offset was down the intercom, without the possibility to choose the plan I want to offet on.
What are the appropriate tools to use?
Note: don't pay attention to the cylinder thingie, it is another derived part I will use later and that should be hidden for now)
Comments
There are several ways to offset. A decently robust way to do it is to use Offset surface along the border faces, then use the edge of surface for anything you like. I recommend staying away from the sketch offset tool, sketches can become unstable very quickly.
Another tool that you will find useful later in the process is Boolean
subtract
. Use this to subtract your template part from the main shape and you can include offset values within the boolean.Learn more about the Gospel of Christ ( Here )
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another options:
try selecting the blue edges and use a 'ruled surface' play around with the settings, to create the surface you want. then thicken to make a part from it.