I've only done it a couple of times, but it worked. I cut a file folder to make my dams. They basically form a "band" around the handle, wider than the fill and held tightly with rubber bands. Where I want the pour I leave an opening and fold up the ends of the paper to form two sides of a pour spout of sorts. I just slip a couple more strips of paper under the rubber bands on either side of the opening and fold up the ends to form the ends of the pour spout.
Here's the best part: Use a hair dryer or a heat gun to preheat the handle and paper right up until the last second before the pour. That helps prevent cooling of the pewter while the pour is filling.
I ended up with a "sprew" right there at the pour spout, but that was easy to file off.