Make new rocker moldings out of thin aluminum which are 1 to 2 inches longer. This will act as side skirts keeping flow out from underneath. Lower is always better for side skirts.
Creative use of black paint could disguise how low they are.
Also, the spare tire area is a mess, a diffuser would be too obvious for your goal, at least a smooth under belly would help.
We're on similar wavelengths here. Several years ago I pulled the stock rocker panels off and made 2" lower/wider replacements with Lexan sheet, and painted them semi-gloss black. I like the looks better, and so far no one has seemed to notice the change.
I had an aluminum bellypan on the car for a year or two, but my unscientific perception of the setup convinced me that the weight (about 30#) was hurting me more than any actual aero improvement was helping.
I've also looked into doing something at the rear, but so far I can't justify the weight of a diffuser. My current direction is to reduce the slight understeer I have, and ignore the rear until it becomes the limiting factor during cornering.