Toys for aggressive/destructive chewers?

I have 2 dogs that are both very anxious and have separation anxiety, and they both are aggressive and destructive chewers. They will also eat things they aren’t supposed to, like pieces of the toys they destroy. So far, the only toys I have found that can withstand their chewing are the black Kong toys, but there is a limited variety of those toys to choose from. I am looking for something I can leave in their kennel with them while I’m gone that they aren’t able to chew up or swallow and preferably something interactive because they tend to get bored. They are a 28-lb Boston terrier and a 15-lb dachshund.