Turns out there are some factor undertray aero parts for the sti of the same year, so I picked up the middle ones.
They are much bigger than they look sitting under the car.
The cover the area between the sideskirt to just inside the main frame rails. The body tapers downward toward the back. These plastic parts fill it in making it parallel with the ground.
After drilling way too many holes and laying down some spraypaint to prevent rust, this is what I ended up with.
Here's a factory pic for reference
And some subaru undertray pron for fun.
Haven't checked yet, but I think my exhaust is hanging too low to bridge the center with a flat sheet of aluminum. That'll probably be a summer project anyway.
Overall, I got longer coasting distances out of these panels, and about +1mpg at highway speed.