Syrup pump for 2L jugs of syrup (not Torani), trim the end to size. For 5:1 ratio start with about 150ml and adjust up/down from there, or about 3 pumps. The colder and more carbonated your water is the less syrup you'll use. Experiment with how many pumps works for you.
The pump does not take up space in the box and you can still order 4L or 8L to fill the box.