It might be more hassle than its worth but a trainer in my club used inner line one of the bigs bags and fill it with water.

It weight a ton but it was the best bag in the gym by far

It would barely move a few inches even when hit by our strongest boxers and it wouldnt hurt the knuckle at all.

it just absorbed eveything put after a couple of months it wasnt as good for some reason but we still talk about it today