Diy rear diffuser

OK so I searched the forum and saw some referencing to a second gen diffuser on our Aveos? Am I missing it? The diffusers I am talking about go in 1 of 2 directions, if u know more than me PLEASE post. #1 there is the plate with holes cut into the bumper to reduce drag of the air caught in it, #2 or the cool looking lower type spoiler with fins. I saw a cool write up an a Subaru owner who made his own and actually sold a couple. Heres my point of the post: would creating a diffuser with mild fins (small) from the rear axle back to the bumper, thus reducing the drag of the parachute effect of the rear bumper increase gas mileage for higher speed driving, such as on the E way, or my second thought of meticulously hallowing out the "show only" black plastic on the rear bumper to allow the trapped air to escape. I do understand at slow speeds it doest do s***, but at 60+MPH, would my MPG and stability increase?And yes they look bad ass!!

Pics would help.