For what it's worth, I, too, have "thirty" and "thirty" and have gone for "x" as the answer. Thirty =xxx in Roman and in mixed notation xxx x xxx=900, the number of the current puzzle. That's a lot of xs!
I did this late, so just come here. Since the rest of the puzzle required adding letters at the beginning or end of words to make new words before entering them in the grid, I thought the single letter to be entered in the centre of the grid should be something that goes in the middle of "thirty" to make another word. So I came up with "s".