the same happened to me aswell sooner or later your ferrets are going to bite throught that wire and judging by the pictures you have some very nice ferrets to loose
to be honest they wont be left alone,as im sure your right they may bite through it ,theyll only be in there whilst im having a beer/working around the garden/playing with the dogs, hour or so each day,but i will be keeping an eye out!
Galvanised welded Mesh works well 20 pound a roll but .well worth it if you want to keep your ferrets