You can use a dog crate but the ferrets won't stay in it!!!!

Being part of the weasel family they can squidge through very small spaces, it is why they were domesticated and bred to be ferrets. Bars of a dog crate are too widely spaced and have too much give in them and ferrets are to smart not to take advantage.

You could resolve this, as it is the space you obviously want, by covering the dog crate with chicken wire or doctoring two or three ferret cages so that they connect.

I really sympathise with you here, cages never seem big enough!!!