Scramble Font

I've tried to type a "blank" tile using any number, as indicated, however it is not working. I'm working on a Mac - would that make any difference? It is still Word but for Mac.

I typed a random character, and then changed the font color to White. Then I highlighted the invisible tile, and did a Border (Custom Border/Shadow/Double Line). I got an empty tile. It isn't perfect and you need to play with the font size of the invisible character, but you can get something close.