An example will help
24 infantry, in three ranks of 8 have a frontage of 160mm and a depth of 60mm.
So three of your bases, has 12 infantry and a frontage of 150mm and a depth of 50.
So pop them on a piece of card or other sturdy movement item, and then either add extra figures, tokens or use a piece of paper to record starting numbers, casualties and keep a track of ranks (loss of ranks has an impact of combat value and once a unit hits half strength – in this case down to 12 models, has an impact on morale)
Alternatively of course, it might be a perfect excuse to start another army 🙂