I found these sponges in a 4-pack at Target for $1.99 (about 50 cents each).
Look at all the pretty colors. My students will love these!
I found these food storage containers in a 5-pack for $2.19 (less than 50 cents each). Make sure you buy a kind with tight-fitting lids.
It's fantastic that these sponges fit these containers perfectly. I found them both at Target. No trimming necessary!
Here are a dozen containers ready for glue.
Cover the bottom of each container with white glue, and lay a damp sponge on top of the glue.
Pour another layer of glue over the top of the damp sponge, and replace the tight-fitting lid.
Allow the sponges to sit for a few days, so that the glue can soak in.
Put what you need to attach on the surface of the sponge and lightly tap to apply glue evenly. Place the attachment and pat lightly to adhere.
Before storing, mist the glue sponges lightly with a spray bottle. Keeping them damp (not wet) will make them last a long time.