Let me tell you what I ended up doing. Previously, I had just covered the vent hole, but it seemed to get air-bound and did not fill all the way. So this time, what I did is take one of the leftover centers from the plug dams and put a hole in the middle, larger than the vent hole, and put it over the hole. When I filled the main hole, plug material rose in the vent as well. As soon as the plug resin reached near the top of the dam on the vent hole, I put another of those centers on top of the one on the ball, sealing the plug material from going any further. I just cut the plug down with a router and it seems to have worked well. Thanks to all for your suggestions.