I used an 1/8" base hole pin, cut down to be just long enough to touch the head cavity. I only cut one down, but i should cut one for each cavity size. As you change cavity sizes, you need a different length pin. But i just cut off the excess if it's too short, or just deal with a small dimple if it's too long.
I also experimented with using JB Weld to plug up the base pin hole all together. I used the clay bar mix type of JB Weld because i was in a "hurry" (read: i'm impatient) because it advertised it would be usably cured in 60 minutes. It worked reasonably well. Some of the plugs got loose after 50 or so jigs.
I'm going to re-do this with the regular two-part JB Weld and see how that does. It says it withstands heat to 500 degress...even if it lasts up to 100 jigs before it breaks down, i'd be happy.