This is what I did, I selected a drill bit (sharp or new) that was bigger than the sprue (channel) and set the depth on the drill press (with mold open and behind drill bit to see how far down the bit will go). Clamped the mold together with two clamps and then drilled down to the stop I set. (You will have to drill halve the way then empty chips out of the mold then drill the other half. Here is the result.
Attachment 83442Attachment 83443