I have never done more than three hours at a stretch - which you CAN do in a cardboard box; especially if you stuff their greedy little guts before you set off, so they are sleepy from a full belly.
However, at some point, your piggies will need to stretch their legs and will start to squabble if the box is too confined. You will have to line the bottom of the box with plastic and put something absorbent between that and the fleece, or you will have a mushy cardboard bottom and a damp seat.
Best use very soft hay, so there are no pokes from unexpected manoeuvres, and make sure that they have their (moist) veg in smaller portions, but more often, rather than water which can leak and cause a mess.
Make sure that the box is covered at night, as flashing lights can frighten piggies and that the temperature in the car is OK.