i have a few remarks. you may not want the dam to reach to cover the whole width of your tires as you'll increase your side wake. i flat plate will have a very broad wake so even if it's smaller than the wdith of the tire behind it it will still send the air around in a clean path,
second you may not want the central dam to be so low, nascar cars have a front ends that coupled with a full airdam will send all oncomming air over the car, where you have the dam it'll send the air around the car.
this graph althoug generic and incomplete sugests that an airdam can be to low and that beyond a certain point, only downforce will decrease but drag will increase again.
(Katz, "Race Car Aerodynamics", p.208)
reasearch on the ople calibra found that raising the center of the airdam also reduced drag (A)
sketch (b) however sugests that the area in front of the tires can should be as low as possible although there's a point where the improvements are minimal so ground clearance is likely more important to keep them in one piece
i don't want to come across negative or as if i know it all, And by all means test what you have because it might just work, but it seems to me that you're puttin a lot of effor into something that might be a dissapointment. and if you have that much plastic you may as well start a modest undertray
, but again, just my thoughts...