In the mammal taxidermy manuals they use a product called formalin
for filling toes and feet of small mammals. It sounds like some nasty
stuff to use and I don't even know if they make the stuff any more.
Do they make it any more or is there a safer product out there to use instead?
They have a product called preserve-it. It's safer and better.
But on another note if your doing squirrels or even larger mammals just inject some caulk to fill the toes.