Thank you, everyone for the comments and the help. I think I found a couple solutions. I talked with the folks at CannondaleExperts.com quite extensively and between the two of us, we found this:

1.
2. There is an 8mm plug on Amazon that I also ordered for the seat tube hole: https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

Either of these solutions should plug the 8mm hole located on the seat tube of the CAAD12. Not I just need a 6mm plug for down tube hole. The search continues.