With the clock ticking,I'd recommend just sticking with your plan.I can think of 'embellishments' but they would really complicate fabrication.
Since it's going to stick out that far,I might recommend 'softening' the perimeter with say,a stiff urethane foam rubber to cushion a blow,should some unwary pedestrian walk into the thing.Like foam knee pad material at the gardening center.
Some sort of simple C-pillar buttress extensions on the top of each side could add some stiffness to the edges of the wing and also frustrate longitudinal vortices attempting to spin up in this region where slower side flow mingles with faster flow coming off the roof.They would make opening the trunk more difficult due to interference.Anything you could do would help.
And a parting remark.If you were to create parallel inset panels which hang below the wing,straight down,of which the trailing edge to the side of the car defined,say,an 7-8 degree angle,you would have a simple boat tail extension which has already demonstrated effectiveness on 18-wheelers and on my T-100.
I'll shut up,you've got enough to do.Happy modding!
