http://www.accuratemolds.com/bullet_...=51-460M-D.png This is basically a copy of the UC Short mold. He doesn't state overall diameter but I'm sure he could make it drop out of the mold at about .506" which could then be easily sized down a few thousandths.
You can buy a Lee .501 sizer and hone it out as needed to get the exact diameter you want (within reason as the honing takes a while to accomplish if done correctly).
Do you know the exact bore size you need? It might happen to be .501 on the nose. The Lee version of that .500 S&W bullet is a double cavity mold that comes with handles included for cheap. It's a gas check design but I think it would seal on the bands if that is the correct diameter. That might be a better choice for experimentation. Lee molds aren't built for abuse, need to be babied a little, but they will drop some nice bullets.