I did what I'm assuming Thrustmaster did with his OC Bats. I used cotton balls. I just pulled them apart into smaller workable bits (each ball into 3 or 4 pieces). I used a Q-tip to push the cotton up into the suit through the waist and the neck opening. I stuffed the entire front and sides of the torso. The chest has A LOT of cotton in it. Before there was a lot of space between the rubber and the body. Not anymore. Feels nice and solid now.
The nice thing about the cotton is that once it's in place it's workable. You can press and shift the cotton around a bit to work out the lumps. However, once the cotton is in there I don't think it will ever be coming out.