Thank you! I've unfortunately had to set this project aside for the near future due to work, life, etc.
As for the jig, I cut two pieces of equal sized 3/4" thick mdf and screwed them together, making sure the edges were lined up perfectly. Next I drilled four holes for the bolts that were going to hold the jig together. I also drilled a pilot hole in the exact center for later operations. I then unscrewed the two halves of the jig. On one half of the jig I drilled a 4" hole, using the pre-existing pilot hole from the earlier operation. On the other half of the jig, I drilled three 3/4" holes within a two inch radius from the center. I then took one of my hemispheres and sprayed the inside with mold release. Once that was ready, I bolted the hemisphere in between the two halves of the jig, making sure not to tighten the bolts too much and crack the hemisphere...again (oops). Also the open end of the hemisphere was facing the three holes I had drilled in the one half of the jig. Once everything was secure, I poured urethane resin into the hemispere through the holes I had drilled earlier. I drilled three holes because I wanted to make sure the urethane would hold securely to the mdf. Of course urethane being what it is, it shrank a little bit once it cooled, but not enough to make a difference. Hope that helps.