With great power comes great responsibility, and with with bellow average modeling skill comes great irresponsibility. 

So I decided to shape some form on lucky13's flat glutes.

Actually this is a remix of two models, a modpart for Lucky13 https://www.printables.com/model/148367-lucky-13-printable-jointed-figure and a remix of 3D “reddit” ass scan https://cults3d.com/en/3d-model/naughties/3d-ass-scan.

The butt model was resized, cropped and modeled to be compatible with lucky13 armature. At this stage it's a little big and takes some degrees of freedom form the hips joints.