How can I slow the cold air coming in a furnace makeup vent?

The makeup air ducts I've seen have been in a U-shaped arrangement where the exit end is high, near the ceiling. This leaves a puddle of cold air sitting in it that doesn't just fall out like it would here, whereas you essentially have a reverse chimney effect happening. It acts like a trap on a sink drain, and the slight vacuum that your furnace creates draws air from the duct as needed.

You could buy a length of 6" insulated flex duct and attach it to the existing outlet. Loop it back up and hang it near the ceiling. I'd also insulate the rigid duct with foil-faced fiberglass. You'll lose less heat to conduction.

Really, the outcome would be the same if you'd shorten the duct to a higher point and install a U-bend up high. There's no value in dropping it to the floor if you're just taking it all the way back up.

You could have success with the bucket idea, but the volume of the bucket outside the duct would need to be adequate to counter the weight of the air in the duct. That would take a large bucket.

Note: I'm not familiar with current code requirements on the subject.