I originally did this in the NX section but the technique should work on most cars, so I'm reposting it here:
My splash shields were in pretty good condition but I never really liked the underside of the front of our cars from an aerodynamics standpoint. Now, I'm no aerodynamics but I do know a little bit about race car aerodynamics and all sedans run some type of flat undertray whenever the rules allow it.
So, with the goal of finding something flat, flexible, weatherproof and above all - cheap, I went down to Home Depot. In the floor and wall department, they sell 4' x 8' sheets of Fiberglass Reinforced Plastic (FRP) for around $30.00. It is about .070" thick, has a smooth side and a textured side and you'll have to get one of the store associates to cut it down to get it to fit in your NX. This gives you enough material for 2 undertrays as the finished dimensions are about 16" x 60" so if you screw one up, you can make another.
When I got home I used a piece of cardboard to make a template of the open area underneath the factory air dam. The actual finished product will mount at the front in the gap between the air dam lip and the air dam support:
At the back, it will mount with 4 bolts into the rear factory splash shield screw holes and a nut and bolt through the rearmost hole in the factory air dam.
When you cut the FRP (you can cut it with a Dremel, jigsaw, etc.) you will need to increase the dimensions by 1-1/4 to 2" along the front and sides so that the tray will sit inside the air dam lip. The lip is not consistent in depth, so you will have to measure the depth at various points to get the maximum amount of material over the lip without touching the part where the lip curls up into the vertical section of the air dam. I am showing the tray sitting inside a spare air dam I have. Do not remove your air dam when doing this project! It never sits straight and true without the support of the rest of the body and chassis.
This shows the relative amount of change in depth of the air dam's lower lip:
Notches will have to be cut to clear the center crossmember and exhaust. I originally thought I had to cut a notch for the A/C bracket but the tray actually will sit below that. This picture shows the notches, the bolts I used (I had some spare 10mm x 1" long bolts that were left over from one of my other NXs) and also the 3/4 to 1" long sections of rubber hose that the bolts pass through. These bits of hose are used as spacers between the tray and the threaded bolt holes in the chassis to get the tray sitting as level as possible while still allowing the bolts to be tight.
Don't forget a small notch at the front of the tray to match up with the divot in the air dam under the driver's side fog light.
This is what my finished piece looks like.
Yes, it installs smooth side down. The textured side has an almost polished finish and so far has been resistant to oil, etc. It stops right in front of the oil pan, so that will still get plenty of air flow to keep the oil at the right temperature.
After playing with the bolt holes I had drilled in the tray, I was able to get this bow out of my tray. A grinding stone on my Dremel let me slot the holes out a little bit. You can also play with the heights of the rubber hose pieces to help level things out.
Take your time, measure twice and cut less rather than cutting too much and making it fit poorly.
Who's got questions?
