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.

With a single strand of insulated copper in my arrow from the back of the insert to the nock, I added 374gr to my 420gr arrow (794gr total). Overall the FOC stayed about the same (~11.5%).

If you go this route or decide to play around with this, I would recomend putting some heat shrink or electrical tape over the bare end of the copper wire so it does not eat away at the inside of your nock as you shoot it.