I'd avoid being too creative with the sound... you want people who read it to immediately be able to read the word out loud without stumbling, you know? You can make up a word, of course, as onestepp says, but make sure it's crystal clear and easy to read out loud because that's what tired parents will be doing with your story every night!
It's up to the editor/designer whether they put the word in upper or lower case in the book. If you want it to have emphasis you could cap it up but you don't have to.