3D CAD DESIGN Help - iPhone Case

I have a small 3D design project I need some help with - I think it is quite a simple job, although I cannot be sure. The attached design is CLOSE to what I need, but not perfect. I do not have the design skills to complete the job myself. This design is a case for an iPhone. On the back of the phone, there is a rectangular slot, to hold liquid. However would like to extend the back shape of the case, so you cannot tell that this extra container is there. Basically, I would like this rectangular container to blend into the case, rather than stick out from the back. Perhaps, rather than being rectangular, this slot would essentially be the same shape as the case. Is this possible?' I hope this makes sense! Thanks, Dom