Do you have a picture? I'm trying to picture how they cut off the bottom of a center draft. Anything you do is going to have to have a hole through it for the draft tube down to the base, and the font will join around that. Look at how an intact one in a store is put together.

You'll also need a flame spreader, which I'm going to guess is missing. The Guild of Lamp Researchers can tell you exactly what you need if you post a picture on their board.