Donna,
Here's what I did when faced with a situation similar to yours...
 
A couple of months ago a bought a large cage.  I suspected that the bars may
not be quite close enough together to keep my ferrets in, but the price was
right and there is not exactly a lot of choice where I live.
 
Sure enough both ferrets could escape, so I covered the cage with a wire
mesh, bought from the hardware store.  I cut the mesh to the sizes of the
cage sides, and top and attached with plastic cable ties (with the excess
trimmed).  I am very pleased with the result and it was very inexpensive .
I used a zillion cable ties as I spaced them very close, but in hindsight I
could of gotten away with considerably less.  The cable ties are pulled
really tight, so it is not possible for the ferrets to chew on them.
 
I hope that may have been of some help.
 
David and the upside down (Down Under) ferrets
Thor - Raisin?!? Did someone say raisin?
Rumpatumpskin - must...bite...toes
[Posted in FML issue 2380]