Are you planning on running endurance racing? That's the only way you'll see any significant advantage, even if you can pull this off.
Vehicle aerodynamics are as close to a black art as any science can get. companies spend millions in wind tunnels, etc. to work out tiny details of this. I kind of doubt just slapping some sheetmetal on the bottom is going to work and may well cause other consequences like overheating, etc.
whatever you do to the bottom, the car's still a barn door up top, with a huge gulping grille and rather large frontal area for size.
2014 Premium GT, SGM, Brembo, Auto, Tech, Comfort, etc. GT500s, Steeda UCA, VMP auto N/A tune. 100 shot nitrous on BBR tune. Best e/t 11.38 at 120.85
2012 Premium GT, Candy Red, Brembo package, 3.73, MT82, Comfort, Tech, et al. Procal tune, Roush UCA, UMI poly LCA, GT500s, Steeda red bracket, Hoosier 28x10x16 bias drags. Best e/t 11.91 at 115.23 RIP