Could you put the dogs on chains/tieouts when you aren’t there to supervise? When I have a young dog that needs time to chill out I put them on the chain with a big meaty bone. They learn really quick that chain = chill out time, and off the chain = fun time.
I would definitely not give them plastic jugs to chew, that’s a one way street to an expensive intestinal blockage. Plus you’re teaching them that chewing on plastic is okay, which is reinforcing the chewing of the ecollar boxes. The more they practise getting to chew on the collars the harder it’ll be to get them to stop.
You could try BioThane or PVC collars, pvc is usually what I see sold with collars. I use biothane for my dogs collars and it’s tough as anything, even when they’re roaring around play fighting, hunting, etc. two of my dogs play by hanging of each other’s necks, and there’s not a single tooth mark in either of their collars.