"sloping back" is correct and usable in written English.
You can use it to describe the shape of something that has a backward slope. For example, "The roof of the old barn sloped back sharply."
They lacked the mammoth's sloping back and high-domed head.
They had a sloping back with a single-domed head and curved, twisted tusks.
The sloping back yard opens up to a pristine lake, with no other houses in sight.
These particular rocks have a sloping back that can be walked up, but the other side presents a steep vertical challenge.
Mr. Porsche also modified the body of the 356, rendering the signature sloping back end and extended hood into a sleeker silhouette.
The dog judged to be best German shepherd looked to be deformed, with an oddly sloping back and weedy, wobbly legs.
Yesterday, thick tanned hands that once lifted heavier loads as laborers across the city gripped chalky balls and gently slapped sloping backs.
Suddenly, she could see that they were in fact pyramids, with steep sides, thickness, broad, sloping backs, and an incredibly complex three-dimensional construction, which changed, collapsing and rising and collapsing, very quickly.
Familiar elements like elongated speaker grilles, textured volume rockers, gently sloping backs, and highlighted camera pods seen in those earlier devices all make appearances on the One, but HTC has clearly upped the ante in terms of quality and construction this time around.
And with that he slopes back up to his room.
His head dropped as he sloped back to his corner.
