How do you write exclamation points in vertically written Japanese?

I am handwriting a Japanese story for school, and I can't figure out which is the correct way to write exclamation points. I know there is not officially an exclamation point in Japanese, but it is used all the time and it is in my story. Once I saw an exclamation point written vertically and it was slanted to the right so that it was diagonal, pointing to the top right quadrant of the character. Is this right? Or should it be straight up and down, and just to the right like all other punctuation? Thanks!