Exterior Window Casing/Sill Problem

[Chris Grantham]

I am trying to set up some windows with a deep thin casing to mimic the attached photo. It's pretty common detail these days and for the most part I have no problem getting the desired effect at least on the head and jambs, but the sill is giving me fits. I have tried all different configurations with using a sill, or not. Using the bottom frame at this sill, and this is closest I can get. It seems like my only option with this is to model the casing separately. Does anyone know how to adjust that bottom sill?