Where to put upper leggings part on 3D armor?

So in vanilla armor, there is a part of leggings above the legs that remains still while the legs swing. I am working on modelling a custom, 3D armor model and texture, but I don't know what to do about the waist armor. On the menu for a custom armor model, there are only options for the left and right leg. I want a 3D armor that resembles vanilla armor, but it won't look right without the upper leggings part and I'd like to add custom model parts for that area. Am I missing something, or is it just not possible? Thanks in advance.