Aloha from Maui!
I handle 12 K-9s for a security compnay here and they are having a problem with several being very aggressive chewers. Any body found a chew toy that takes a whhile to chew down. Yes I've tried black Kongs...POOF 1 day..gone!
My police dog is very much a power chewer. I haven't found anything he can't destroy. What I have found, however, is that he destroys the large kongs much much slower than the XL & XLL/King/whatever kongs. For some reason the size large gives more and he doesn't get the chunks of rubber off nearly as fast. In fact, the one he's been chewing on lately has lasted at least a month. I talked with a rep from Kong and he said he's heard the same thing from other handlers....the smaller ones actually last longer than the giant kongs. Just a thought in case your dogs are getting the giant size ones. Otherwise, I haven't found anything else. Good luck! Maybe someone else will have a new idea