Not sure if I understand your question, but I give it a shot. I think you ask if it is possible to create the box and remove the grove to mount the box to the rail.
I personally would draw the side of the box as a master sketch.
Basically the bright yellow side facing to the viewer in your picture without the mount. Then I would pad the face of the box section.
The result is a block you can put onto your rail, but it of course lacks the opening. This can be achieved by a simple function named “thickness”. The name is a little confusing, but the basic function is to apply material on the outside to create a box, BUT if you select the checkbox (Apply thickness towards the inside or so) you simply specify the wall size you want and add on all faces you want to be open to the list.
Next select the lines of the mount bracket and a pad. I added a length reference to the master sketch, so both pads are the same and you can change it.
The result will be a container like above in just three steps plus a sketch. I just did a rough outline without any real world measurements and the connector will be wrong, but you will get what I was to achieve here.
All you need to do at the end is to add chamfer and fillets. There are multiple ways of designing stuff in CAD. There is no right or wrong. Well, with FreeCAD editing on faces is wrong and especially the official version of FreeCAD is buggy like hell in reference department. Google “FreeCAD TNP” and you will see. I used this version made by RealThunder to create the example and can recommend it. It works like a different application.
I used a feature names master sketch. I basically drew multiple lines into one sketch. The entire sketch cannot be padded, but you can select multiple parts of it, create a binder and pad only that binder. With the real thunder version the binders will automatically created when padding/pocketing shapes from the master sketch.
The master sketch way you can have everything on a plane in a top down view within a single sketch and you can use what you need at a time. e.g. two circles in the same sketch. One is for the screw hole and through all, while the second is only a few mm deep to take the screw head. No need to reference the anchor point of the circles twice, which you would need with separate sketches.
ExampleBox.FCStd (34.9 KB)