While I waited for my sugru to arrive, I decided to use a reporter's style Moleskine notebook I already had, and considered how to "hide" the magnet so that it wouldn't contact the screen directly and potentially scratch it.
This is what I came up with.
List of materials:
2 packets of sugru
small magnet (mine came out of a cheap refrigerator magnet, but it does have to be a solid one, not magnetic tape)
one large sheet of craft foam
quick-drying glue (superglue, etc - I used Titebond)
duct or gorilla tape
recommended: a ruler
I apologize in advance for the quality of the pictures.
Step 1: Cut all the things.
The notebook isn't thick enough for everything we're going to add to it, so we need to make the binding thicker.
First, cut the paper away from the cover, front and back.
Next, cut straight down the middle of the binding itself, making sure to leave some material on both sides.
Take the front of the notebook, and use it to cut a piece of the foam sheet to match, as exactly as you can. The foam only needs to reach up to the inside edge of the binding. If you leave it longer, it will make it difficult to close the cover later.
Cut a shorter piece of foam, as well. Imagine it as only the lower half or so of the first one. This will make a small pocket for cards, etc, as well as covering the magnet.
Speaking of which, place the magnet over your tablet to find where, exactly, it works. On the Nexus 7, it is just slightly above the pogo pins on the lower left side. Match this up to where the cover will come down (it's best to mimic the cover actually closing, to be sure to get the right side), and cut out a hole in the larger piece of foam where the magnet will go.