Sorry in advance if someone has already mentioned this method, but I don't recall it being discussed and I used the search key.
When pouring my resin baits, ballasting was always one of the hardest factors to nail down. Plus filling the holes was always a pain.
A few years back, I came up with a method that I now use on all my resin baits.
I pour two separate resin mixes into the mold. As a rough approximation, I pour the lower 2/3 of the bait with no microballons, thus making it more dense that water. I add microballons to the top 1/3 making it less dense than water. The bait always orients vertically without any added weight, drilling and filling. There is no line between the two different resins in the finished bait. Only a slight color variation. An additional benefit is that the bait ends up weighing less than if I use lead for ballast. sometimes as much as 10% less. A big deal on a 10" bait.
I know this is not the greatest explanation, but I hope it might help.