Living in upstate NY, a guy found a big block of dock foam floating in a small stream. He about had a hernia getting it to the truck, being water logged, but 4 of us shot that thing all summer long w/ broadheads or field points.
Never shot out! It was BIG and THICK!
I think the durability came from being water-logged! We had to keep a old towel beside it to wipe the water off the shafts each shot from 'static cling'. Worked good though...but where do you find old water logged dock blocks?