To create the feature, try these steps:
On the right plane I create a sketch that very closely followed what you had already laid out. The Curve - Split command is used to split up the model faces. This is exactly what you had done earlier to separate the areas getting the rubber grips.
Now delete the faces that were created above.
I created a new sketch on the Right plane. It is a simple line between the "points" at the tips of the shape created in step 1.
Ideally this step should have been made back in step i, but there was an error, so I just split the step into smaller steps.
Now delete the faces that were just created in the last step.
Look in your feature tree and you may see that two surface bodies now exist. There was a tiny little sliver created in step 3 and step 4. I was unable to select it by editing step 4's delete face command, so here I had to use the delete body command.
Now for the tricky part. You need to either make a reference plane at an angle, or you can use a 3D sketch. I used the 3D sketch option, but both will give the same result.
The sketch is a three point spline with the end points defined by the "points" of the shape made back in steps 1 & 2. The third point just needs to be around the middle of the spline.
Drawing a 3D sketch is a little complicated at first because you HAVE to be sketching in the correct orientation.
So start a 3D sketch, choose the spline tool, and look at your mouse cursor. You should see a "XY" next to it. If not, press the TAB key until you do. Sketching in the wrong orientation leads to nothing but failure and headaches!
While still editing the 3D sketch switch to the Front view. Now click and drag the middle control point of the spline in the "X" axis until it is where you want it. You can add dimensions and reference geometry to help lock everything where it needs to be.
Now use the Fill Surface command to fill in between the 3D sketch and the "flatter" of the open edges of the hole.
There are other surfacing tools that will fill this hole in and you should try them all out. I usually find that Fill works well when there are only 2 or three edges though. I set two of my boundary settings to tangent, but it did not make much of an improvement over the default Contact setting.
Ideally, there would not be sharp points at the ends of this opening, it creates a "singularity" in the software, and can make it difficult to obtain nice blends.
Lastly, another Fill command to fill in the remaining hole. Instead of CTRL clicking all of the little faces, try right clicking on one and choosing "Select Open Loop", it is fast and gets little pieces that you might otherwise miss.
Also turn on the Merge Result and Create Solid options.
Due to the "singularities", this is not a very good patch, but it captures the shape, and it looks fine in renderings. In reality the "marketing" guys will want to make a hundred changes to it before production, so it may be best to get the overall shape quickly as a visual. If they really like the shape, then it can be added in a better way for the production models.
Give it a try and see if it works for you.