Our trainer has told us to err on the safe side and nothing is to go into the crate if you aren't there to supervise (even a super durable black Kong/Nylabone/etc.). Not even a bed or towel. Seems sort of harsh, but especially since your dog is a chewer, what happens if your dog chews the top off that Kong and swallows it, and you come back home to find your dog dead from asphyxiation? Or you leave a Nylabone in there and come back home to find half of it missing and have to take your dog to the emergency vet because you're not sure if it's going to puncture his stomach?
Is your dog crated the entire 8 hours you're at work?