Because of the way
Thicker Ass and Legs reshapes the butt, radial alignment between the right thigh and back layers is terrible. As a result, the areas of the layers on which I can draw are determined by the range of motion I expect from SDT-chan. I can't draw above a certain line on the thigh, or draw below a certain line on the back, for use with the vanilla positions, unless I add a lot of material with no details on it to cover up the problematic bits I drew which rotate into incorrect places. Basically, anything skimpy must be ultra-low rise, or it looks terrible.
These pants tend to look better the more bent-over SDT-chan is, which makes them work best with large bodies. As she kneels upright, they begin to appear as if they are not flush with her stomach. Big bellies can help fix this too.
Thanks to @stuntcock for resolving the issue of the mask functionality required to use the chest layer properly with Thicker Ass and Legs. There wasn't occasion to take advantage of it with these shorts, unfortunately; the radial alignment problems and the small area of skin obscured by the clothing meant that I didn't have the option to use the chest layer at all. With more scripted masks customized for a particular outfit, I think more could be done, but that seems a bit excessive.