I created this splitter diffuser for a track focused 4G Civic, and it worked well enough for me to be okay with publishing it. 
Theoretically, it can increase the effectiveness of the front splitter according to some sources.

You do need a big printer for it (dimensions in the last picture) and I recommend PETG or high-hardness TPU for it as you need a material with good layer-to-layer bonding.
The STL file is already in the preferred printing orientation. For some reason, printables doesn't want to accept my .f3d file, but I will try to upload it again later.

Please share any images of it on a splitter if you actually print it as we crashed our car before I could take any pictures :(
P.S: It wasn't the diffuser's fault the car crashed.