I'm glad you got your part finished and in Solidworks there are many ways to accomplish the same thing.
As I said before you could have put a block into the assembly and used cavity to define the entire part and not had to mess with offseting, trimming, and extending surfaces, but each to his...
Timelord,
You can throw a block into your assembly that is bigger than your assembled mold cavity but smaller than the outside edges of your mold. Then run the cavity command to remove all the edges of the block to make your finished part. It is actually pretty simple. Once you get your part...