gluing/taping polycarbonate underbody?
 

I want really simple/cheap aeromod, thinking of partial flat underbody which is not permanent. Can I just glue/tape it on? Don't like screws.
I know I have to avoid exhaust system so it will be partial not full underbody.

How much fuel efficiency does this gain?

If anyone has done this in S. California, maybe they can do mine.

gain
 

Back when bellypan studies were being conducted in the more modern wind tunnels,the underside of the car was attributed with about 12% of overall aero drag.
With a nicely executed pan you could cut that 12%.
At 55-mph,this would translate into a 6% mpg improvement.
At 70-mph,around a 7.2% improvement.