I'm missing something…"I have down (split in ounces)…"
Just laying there on the table?
In any case, a piece of mosquito netting taped over the end of a vacuum cleaner wand will allow you to grab and compress down into a wad. Place a ziplock over the wand, turn off the vacuum, etc, etc.
If you are "collecting" a lot of down, add a removable tube to the wand – like a toilet paper tube. That will allow you to accumulate quite a bit. Find something to work as a plunger to push the down into a ziplock.
Or provide a few more details.