I put this in alot, so here it goes again.
Buy some 1/4" diameter rope. Braided nylon rope will add about 100 gr. Cotton will add about an additional 100 gr.
Check the diameter inside first. Some of the newer carbons are getting really skinny again.
I cut the rope about a 1/2" short, tape the front end with masking tape or blue painters tape. Then tie a small slipknot in 6# or whatever monofiliment fishing line I have. I take a paper clip and cut off the straight side. Tie the line to the paper clip and thread it through the insert and down the shaft (don't try the other way, been there hit my forehead for it
). Take off paperclip and slip onto rope. Pull through. If you need some extra room for the knock, pull the rope out ( rat tail file works good for this) and cut to length, repull and put knock in. Shoot!
Otherwise, they sell weight tubes for pretty cheap. I think 3 Rivers goes up to 8 gr. per inch. BUt the rope will do a ton of arrows and costs about $5.