What kind of ultralight fill? If it is the fluffy kind, yuck. You will have to use a dowel and push it through a funnel. It’s a pain to do ultra light fill when the bag is half open.
I have tried all sorts of ways to make filling easier.
You would be better off emptying it and filling it with polypropylene beads to get a medium weight bag. If it is the fluffy stuff on top of sand, you’ll get a weird fill and a little bounce which usually isn’t the desired effect off a bench.
Feel free to PM me your phone if you want to chat. We deal with this stuff all the time.