The drafty door between our unheated mudroom and our kitchen has been driving me nutty!
The tile floor was too cold for bare feet, and I could almost see the money from our heating budget seeping through the crack!
This morning, I finally fixed it. Here's how I did it.

First, I measured the door in units of empty paper towel tubes. Since they had to fit inside one another to form one long tube, the total length (of three tubes for my door) had to be a bit longer than the width of the door.

I fit the three tubes together, folding one to fit inside the other, forming a "supertube". These were attached with a ring of glue from a hot glue gun, along the inside of the outer tube.
Using a chopstick and a broom handle, I stuffed the supertube with plastic grocery bags, packing them tightly.

On the door flashing, I placed a super-strong magnet on each nail. Then, I put some hot glue on the magnets, and quickly pressed the stuffed supertube onto the glue-y magnets.
The result is a supertube that attaches to the (nails in the) flashing, magnetically.

This project requires the strong magnets, because the flashing is aluminum (not attracted to magnets), so the tiny nail-heads are the only metal for holding the whole supertube. In order to work properly, the attraction has to be strong enough to hold up to frequent door opening.
For doors without flashing or metal, see the next post for what to do.

When opening the door is tested with a full length supertube, it will knock the supertube off, due to interference from the corner/end piece.
Trim the end piece just enough to allow the door to open easily, allowing the maximum draft-covering length to remain.

I needed about 3/4 of an inch of clearance.

Then I covered the supertube with fabric, using the glue gun. I was careful to leave only one layer of fabric over the magnets, to avoid interfering with their hold capacity.

This was a very quick project, using materials on-hand and about 10 minutes (including photographs.) The frog fabric was from my stash, purchased on clearance at about $2/yard, and I used less than a quarter-yard. Scraps of sheets/tablecloths would work nicely, too. The total cost was under $2.
I wonder how many minutes of New England Winter it will take to recoup my costs from the heating oil bill?

It works very well, and our bare toes are grateful!

Stay warm, everyone!