I just bought a 1994 honda civic and the person I bought it from has drilled 4 1 1/2" holes in the rear bumper to reduce drag but he said there should be a trim ring to be inserted into the holes but I cannot find any.
If anyone knows of a place to get some aftermarket vents or trim type rings it would be a help.

 

You could probably find some at a marine/boat store or maybe even at a truck stop/accessory store.