Just a thought, haven't tried this technique but in theory it seems like it might work.

Take a piece of plywood, plastic, metal, or whatever, cut it into the diameter of the bucket. Then cut a slot down the center. (Make it look like a big coin slot.) Feed the end of the bag through the slot, gently push down to remove the air, fold and seal bag. Then carefully remove this piece and move on to the next bucket. (Might consider adding a handle or two for that step.)

Like I said, might work, might be worth the effort. Hope it helps.

Badger