Phone case modelling for handhold grip so i can 3d print it.

I have a weird phone, a Doogee S80.
I have a .stl 3d model of the phone case that comes with it.
It is here on thingiverse https://www.thingiverse.com/thing:3231543

I want it modifiying, extending, and a hand loop putting on it., as per the pictures, attached.
The website wont let me upload mo9re than 3 pics.

I want one edge of the model, extending by 4cm, and i want a loop putting in from the top left extended 4cm edge, down to the bottom right edge. The loop needs to be 4.5cm off from the body of the phone case, to accommodate the depth of my hand, and needs to have a usable width of 12cm to accommodate the width of my hand. it would be nice if it could be somewhat S shaped, so that it will avoid the camera's field of vision. it would also be nice if on the lower section of the handhold there was a toughen apeture/hole that i could put a key ring through, so i could attach the case to a chain attached to my belt of something?

Thanks in advance