This covers how to make undertrays that work. It might not be the longest lasting (or the prettiest), but with a couple of hours and about $10 in materials (depending on scrounging ability) you can see if it's worth building properly.
Front undertrays are aerodynamic aids that serve to reduce drag and lift. The rear one does much the same. The theory is that the easier the underside of the car cuts through the air, the less positive pressure is present there, and less drag and lift is developed. That's a bit simplistic, but it will do.
The finished product will look like this... Yes the front one has a downward angle, but read step 1 to see why.
Step 1: Assess the situation
The plate currently does three things.
1. Provide a negative pressure behind it to extract air from the engine bay
2. Provide a slightly higher pressure in front of the radiator (to encourage airflow through it
3. Act as a stone guard for the cast alloy sump
I want to keep all 3 of those aspects while improving aero. Which means I have to run a sheet from the bottom rear of the front bumper to that plate. I also wanted to block air from going into the part in front of the wheels.
The rear section is just untidy. The other side is more fuel tank in place of the exhaust. I wanted to cover this, but unfortunately the exhaust meant that I could only block in the middle and right hand side of the car.