I can't remember (should be better about making notes) where I saw this idea, but thought it was really cute and fun.
I bot the Garanimals bath letters and numbers collection at Walmart. I buy the diaper wipes at Sams and you always get a new box with them. I don't need to replace my wet ones box every time so decided to do something else with it. Then I saw this idea.
Now the only thing I found was you couldn't give them all the letters at once as they can't them all in the box. You can store the letters all in the box, but once it starts getting full, the kids had trouble getting them in. So next time I will only give them a few letters at a time. They really enjoyed doing this.
The numbers (not pictured) did fit well in the box. Thinking I will let them place with this for awhile first then we will talk more about the letters and let them learn thru playing with it.