It is a very good idea to have a travel cage on hand in case you need to go on a trip, for long drives, in case of emergency (i.e. natural disaster, family emergency requiring a trip of ferrets staying with a friend, etc), need for quarantine of a sick or new baby, etc.
Get a guinea pig sized cage on CL, you can get them fairly cheap. Hang a few hammocks and put in a small litter box. I drilled holes in the base used zip ties to secure the top to the base otherwise they can escape really easily. I prefer top-access to the travel cage, but it is nice to have both options, esp depending on the space the cage will be in. I zip tied the front access door closed on mine bc it wasn’t super secure and I never use it.
I use a lock crock for water but also use a water bottle as supplement in the car in case the water splashes out. They have a small bowl for food (we go on long trips with them), the litter box is attached to the side of the cage via binder clips that hook to zip-tie loops, and I always hang a few foraging toys, especially for longer drives or long trips out of town.
To attach the litter box, use a drill to make holes in the base and then zip tie loops on the base of the cage that the binder clips can hook onto.
This cage fits perfectly in the backseat of my small car with a little room leftover. I use it for long trips for my two boys and it works great. With more ferrets you may want a bigger cage, but for a small business this works great. I have use it as a sick cage, they have lived in it for weeks at a time when on trips (with plenty of out time of course), and it is amazing for long car rides – they don’t even notice they are in the car.