here are some pics of w/o undertray
and with undertray. purpose of the undertray was to avoid having the large transverse bar facing the flow, so the udnertray is composed of two parts, one kind of deflector in front, and the bottom diffuser.
my plan for this is to perfom tuft testing to check attached flow on the diffuser. (once i will have the boat tail attached to the end of the car this will be even more important!)