First, "old" down bags were made before the new 800-900 down was available. Depends on the old bag - if it is a 650 down (the most common type of bag in the old days) it will be heavy; an elephant's foot made of an old bag will also be heavier than what you could buy now.

Have you ever sewn with down? Very touchy and feathers everywhere! First, you need to have more than "half" the bag to make an elephant's foot. You need the bag to come at least to your waist when finished. For most people, legs plus body to waist is more than half their body length. Second you need to leave about 2-3 inches minimum to finish the seam and put in a drawstring. Because of the feather problem you need to cut the bag above a horizontal seam. This may result in more than waist height.

One trick if sewing with down, wash the bag first and then handle it when the down is partially dry so feathers do not fly everywhere. You have to be careful because if the down is too wet it is heavy and will rip out the internal baffles.

However, if you just want to try it to see if you like the system before investing in an expensive elephant's foot, it may be a good experiment. But do not expect to have a light half bag.