Fill remaining text box slots with X's in Access form?

I'm working on updating a form to enter Batch numbers for products. To scan the barcodes we use 13 characters. The batch numbers are not that long so I want a way to fill the remaining 5-7 spots with a variable, "X" for example. Is this possible with Access or should I just set a rule to make sure 13 characters are entered. I could try an input mask but I feel because the batch numbers are unique and vary, I could use 6 or 8 numbers at any given time. Thanks!