cob as altertnative to expanding foam?

Squirrel!  now that I have your attention, we have had some red squirrels living without paying rent in the roof of the house, and there are a lot of gaps to seal.  The no-kill wildlife expert gave us a quote of over $2500 and would use expanding foam.  I would love to be able to cob together a solution without something that has an MSDS sheet.  Would cob be chew-proof and adequately fill the gaps that need to be filled?  (It's wood house, concrete or fieldstone foundation.  They go in there, up the walls, and then nest in the roof.  There isn't really a way to put the cat up there to see if she would work for us for free, which she wouldn't anyway.

Couple of ideas;
- photos of gaps etc
- install nesting houses outside or on side of building
- fit wire mesh in gaps because blocking may reduce air flow through roof space

Normal cob is not squirrel proof.
Maybe use stainless steel scrubbers as part of the mix?
Maybe borax?

I like the idea of using the idea of using the stainless steel scrubbers.  

In the past, we have used a stainless steel tape so the combination of the two might work.