It’s not practical. String isn’t dense enough. You would have to have a pretty big ball, one that isn’t reasonable to play 5A with. A string ball the size of a counterweight barely weighs anything. However, you could probably get a Duncan Ball Counterweight, or a plastic/wood/metal ball counterweight, and glue or coat it in string or something.
Au contraire. I use a string ball counterweight that has a diameter of about 1.25 inches, and it works fine for me. Some of conventional 5a tricks are certainly next to impossible to do with the lightness (such as the Butterfly), but most are fine (at least in my limited experience, sorry I’m not much of a 5a expert). I’ve also found it has opened just as many doors as it closed. It’s fun to throw on a light counterweight and just start making stuff up.
You can use a set of washers or nuts around a hollow straw like piece and do the string around that. I’ve done that before and you get good weight with it and you can use a lot of string in the process.
Gosh, now I feel kinda bad about throwing all my used string away. I will probably start saving my new Expert Strings since I haven’t thrown any away yet.