I use the aquarium tubing. But I add a twist.
I squeeze automotive silicone into the tubing, that has been cut into short pieces. I allow it to dry, and cut it to the desired weight. I use a digital grain scale to get the weight I want.
With the arrow the desired length, and the insert installed, I drop the undersized tubing into the shaft through the nock end. To hold it in place I use a short 1" piece of rope that fits tight in the shaft. I push the rope down with a piece of round steel that will fit inside the shaft, like a muzzle loader. The tight fitting rope keeps the tubing in place, and prevents it from rattling.
I have had to "repack" the tubing a few times when it loosens up, and even was able to remove
the rope and tubing to try different weights.
This method keeps all the weight forward which is what I wanted, and may not work for everyone. If it changed the spine of my shafts I could not tell
and it gave me the best flying arrows I have ever had. I am going to experiment with some new shafts
this winter, and I`m going to eliminate the tubing and just inject the silicone before the insert is installed. It will take a while for the silicone to dry inside the shaft, but it will I`m sure. It takes about three days when I put it into three inch pieces of tubing.
Seems like alot of work, but my carbons have alot of miles on them, and are still going strong.