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.
Step 2: Glue All the Things.
First, glue the larger piece of foam down to the front cover of the Moleskine. Be sure you've placed the hole for the magnet on the right side.
I found it best to put the glue on the cover, and press the foam down to it. Be sure to get right up to the edge so the foam doesn't peel up, and smooth it out carefully to avoid wrinkles.
Next, glue the magnet down in its spot.
And finally, glue the shorter piece of foam into place over the magnet and around the sides and bottom edge. (I expect to put a few business cards in mine, I doubt it is substantial enough for much more than that.)
Sorry, I missed a couple of pictures for this part, as I got carried away.
Step 3: Tape All the Things.
First, make a band at the top of each part of the cover. Overlap the foam on the front a little. Be sure that the tape extends beyond the edge of the original binding at least a little on each one. Use your thumbnail or a hard edge to push the tape into the edges made where the foam meets the back, and where the backing ends and the thinner binding begins, so that it will be more foldable.
Next, unroll your tape about 9 or 10 inches and lay it out, sticky-side-up. Place each part of the cover so that some of the band of tape already there overlaps onto the new piece, and use a ruler along the edges of the covers to make sure they line up straight.
Once they're placed correctly, fold the free end of the tape up tightly to the insides of the covers, showing you how much more you must cut off. Cut the last part of the tape so that it meets the edge, but overlaps as little as possible (again, if we make this too thick, it won't bend very well).
Step 4: Sugru All the Things.
I used about a package and a half of sugru, in the end.
Take small balls of sugru and press them down to make flat "buttons" where you want your clips to go. This takes very little material, you're just making the bases for now. I decided to make more substantial corner clips at the bottom, and smaller side clips at the top, but I had to be careful about placement at the top because of the power button on the right side. The first picture actually shows them too low, I had to move them.
Next, make 4 small "sausages" of sugru. Press each one, end down, to one of the buttons, flattening out the base. Then shape it to lean out at an angle until you position your tablet.
Once all four are in place, wrap your tablet in saran wrap and place it gently where it will go.
Shape each piece of sugru up and over the edge of the tablet. Be careful to keep the clips as smooth and even as you can.