How to center each letter in each box of a combo field?

Before anything else, sorry for my English - not my first language... 😅

I've one of those forms where you must to fill each box with only one character. Obviously I used combo fields to to fill them, and I must say it works quite well. However, since each box is separated by a small gap, some of the letters are not exactly centered in the squares. 👇

What I want is for each letter to be perfectly centered to the respective box.

Do you think you can help?

PS: I don't understand anything of Javascript