Slow Feed Nylon Hay Bags by Derby Originals
I use this bag. There's literally no where they can get their hoof stuck and it secures safely to the fence. It holds about 15-20 pounds of hay if you really, really stuff it. Yes, it is a little pricey, but it's a lot safer and really slows down their consumption. With this 10 pounds of hay last my colt 10-12 hours! I have heard cases of horse biting the baling twine and either getting it stuck in their teeth or breaking it. No way they can do that with this bag. Slow Feed Nylon Hay Bags by Derby Originals
This bag is similar, but I don't know how much it holds or the quality of it.