Here is Kamm's last 'Kammback' from the late 1960s.
You can see that it is perfectly integrated into the body with seamless transition.The air needs this 'perfection' in order to protect the boundary layer,which determined whether or not you'll have separation.
Your cardboard mockup is more like a box-cavity,and for it to work,it needs to be as low as the bottom of your bumpers bottom,and it needs a 'floor'.It cannot be open on the bottom unless there is a back on it,and the back is airtight.
The trailing edges of your box would be at the same place as it it were a perfect boat tail Kamm extension.
The top edge would be like on the XL1 or 'Template'
Your plan-view edges would want to follow a pathway as on the 1957 MG EX 181,with Cd 0.12.Just scale everything to the drawing.
Your side panels must mimic the curvature of the SUV's body when looking from behind.
It's more work,but the air doesn't allow any shortcuts.Ever!
Here is Kamm's lowest drag K-car,Cd 0.23
This is where you want to go.
