Whilst I would need to have a better understanding of your current set up, an alternative idea could be to utilise groups to identify each month of the year, and list your customers as items in each group. You could then incorporate a status column to identify whether customers have paid you or not.
Alternatively, you could list each customer as an item and use subitems to mark each month. There you can again set up a status column to mark whether the customer has paid you.