My kids have their own little art corner where they work on their paintings and such. They always do a rough sketch in pencil, draw over it and then erase. Almost inevitably I hear someone screaming at another someone over where they left the erasers. Then we spend the next half an hour looking for one. I recently came up with a simple solution:

A Hair band or two around the pencil cup does the trick. Any elastic will do. One band will hold several erasers or whatever small object you want on hand.

See how happy they are hugging the mug :)

We also use this method to dry damp paintbrushes to keep them from getting water damaged while laying flat.
I hope that you find this as helpful as we do!