CPES is good, but this product is specific to rebuilding transoms. It's called Seacast pourable Transom material.
Here is the link:
http://www.transomrepair.com/
The stuff really works, too. Definitely stronger than plywood. I wish I had known about this stuff back when I had a bum transom in my old boat.
I'm very impressed with the stuff, and how easy it is to use. It turned a junk boat into a very solid vessel that I wasn't even nervous to go out in. It has the potential to save a lot of time and money and is definitely a permanent repair.
TH