As Max said in his post, you really shouldn't mix numbers & text. However, to
get the appearance of what you want, you might try this setup:
Selecting every other column, change column width to somthing small, say
0.5. Repeat with row height, making every other row about 8.25. Set the other
rows/columns to heights/widths appropriate to what you want for crossword.
You should have a sheet with tiny squares, big squares, and rectangles.
Ignore the rectangels. Selecting a group of four cells (1 tiny square, 2
rectangles, 1 big square) format cell to have a borer around edge. This will
give appearance of a single square, creating base for your crossword. Copy
this formatting to other cells as needed. Now, you can place a number in top
left corner, and text letter in bottom right corner.