In article ,
Christopher Tidy wrote:

When I was about 8 or 9 years old I really, really wanted a stone
polisher. I think they were rather expensive and also (probably wisely)
my parents thought that I would be bored having to wait months for
results, so I didn't get one for my birthday. Anyway, I woke up in the
night and it occurred to me that a stone polisher (or rock tumbler as
some people call them) would make a great home shop project.

Round & rubber is better than polygonal & steel. The setup at the local
serious rock shop back home was rather handy, if a bit large for a kid's
project - they had a set of tires on powered rollers - no fuss with
sealing the drum, and you could pull out a few to look at the progress
without it being the huge production it is with the small drums. Perhaps
for a smaller one you could scrounge a few lawn tractor tires - they
used car tires.