One way to make a pretty darn heavy carbon, without screwing up the spine too badly is to use a piece of insulated solid copper conductor out of some romex wire. Similar to the way some use string trimmer line as weight tubes.
Another thing I've found that works well is aquarium airline. I had some that fit perfect in my gold tips. It was tight enough that it didn't rattle or move and since it's hollow I could add another item to get them even heavier if I wanted. The airline was tight enough to actually increase spine a bit, which is good when you're adding a lot of weight to the tip. The stuff is super cheap, which is always good. The airline I added, was about 250 grains.