Pete C. wrote:

You might want to try some of that "fixturing alloy" you can get from
MSC/Enco/McMaster. Since it melts at such a low temp you might be able
to directly cast it into the plastic wheels.

Yeah... but you'll still need to machine it in place to balance.

And don't let the cub scout handle the stuff unsupervised - same goes
for the lead really.

4 wheel weights of lead may easily exceed the 5 oz limit... what about
using some more ordinary engineering material such as free-machining
brass?