You can simply create a plane offset from your surface and draw e.g. a rectangle that's the size of your fixture. Now extrude up to surface but un-check the merge bodies checkbox. Voila. You can hide the body of your original part to do your CAM work.
I just did the exact same thing a few weeks ago to hold a curvy part at an angle for drilling.
Please forgive the sloppiness of my design tree. This is kind of a quick and dirty project and I'm still learning SW
In addition to Andy's method, there are several other ways to do it. You can use the Cavity tool. you can use an Indent feature. You can use a Cut with Surface, with the surfaces generated by Offset Surface or Knit.