Looking for a second-hand Rock Tumbler

I’m looking for a reasonably-price rock tumbler for my 8 year old who is a mad keen wanna-be geologist. Is anyone able to help please?

If you’re blessed with a couple of DIY genes… here’s one I saw a while back.
$15 warehouse 3 speed fan motor, 10l bucket and some skateboard wheels… Might want to restrict 8yo to just loading and unloading bucket.