Ah, the concept of headology is put to use. Or in other words, just use whatever you´ve got if you want to do an invocation and believe in it:
"We conjure an abjure thee by means of this---" Granny hardly paused --- "sharp and terrible copper stick."
The waters in the boiler rippled gently.
"Se how we scatter---" Magrat sighed --- "rather old washing soda and some extremely hard soap flakes in thy honour. Really, Nanny, I don´t think ---"
"Silence! Now you, Gytha."
"And I invoke and bind thee with the balding scrubbing brush of Art and the washboard of Protection," said Nanny, waving it. The wringer attachment fell off
I love how practical minded the witches are. Even though the summoned demon isn´t quite as impressed as I am:
"My name is unpronounceable in your tongue, woman" it said.
"I´ll be the judge of that," warned Granny, and added, "Don´t you call me woman."
"Very well. My name is WxrtHltl-jwlpklz," said the demon smugly.
"Where were you when the vowels were handed out? Behind the door?" said Nanny Ogg.
"Well, Mr -" Granny hesitated only fractionally - "WxrtHltl-jwlpklz, I expect you´re wondering why we called you here tonight."
"You´re not supposed to say that," said the demon. "You´re supposed to say-"
"Shut up. We have the sword of Art and the octagram of Protection. I warn you."
"Please yourself. They look like a washboard and a copper stick to me," sneered the demon.