Douglas Harper's Etymology Dictionary
soap-box
also soapbox, 1650s, "box for holding soap," later especially a wooden crate in which soap may be packed; from soap (n.) + box (n.). Typical of a makeshift stand for a public orator since at least 1907. Also used by children to make racing carts, as in soap-box derby, annual race in Dayton, Ohio, which dates to 1933.
Usage examples of "soap-box".
And after the guests were gone, I learned how he had met him, passing down a street at night and stopping to listen to a man on a soap-box who was addressing a crowd of working men.
It is very tiring to rap out letters of the alphabet, and she was afraid the soap-box was slipping.