I'm a little confused as to how/where the ferret is getting under the
dishwasher, but you're right, it's not a good place to be.  (Why is it
that ferrets always want to be where it's not safe for them to be?)
 
Our dishwasher sits flush to the floor and sides, I'm not sure what you
mean about an "arm" coming out.
 
On the other hand we've had a problem with the gas stove which has a 2"
gap under it.  We solved the problem with a curtain rod.  We took a cheap
(under $3 at Wal-Mart) white curtain rod, bent it out with pliers to be a
flat surface, and screwed it into the bottom of the stove with sheetmetal
screws, leaving a gap of about 1/2" to allow for air circulation.
 
You might want to see if something like that would work for you, or
else maybe the rubbery baseboard "molding" that can be purchased very
inexpensively at any home improvement place.  It's heavy enough that I
don't think a ferret could push by it, and still somewhat flexible.
 
Another option would be heavy duty screen or chicken wire.
 
Good luck!
 
Anne
[Posted in FML issue 4803]