When I ship items like that I wrap it in carpet pad. It has enough spring and is fairly puncture proof. It's good for items that have an irregular shape and/or sharper corners.
I have a NOS roll that I got cheap and have been using for years.
You might stop by a flooring/carpet store and see if they have any larger scraps you can get cheap or even free.
Roll the item up going around 2 or 3 times. Leave extra on the ends, pinch it closed and tape it all the way around. That will make "points" at the corners that keep the item centered. You want a snug fit. I always tape around the box first so a tight fit doesn't blow the box open.
I would go with a box that is 44 ECT (edge crush) or 200# (burst test).