Anyway, until someone does something better, here's the info on how I did it at least.
Step 1: Build or Buy a Cover/case
Step 2: Locate Your Magnets: Polarity Is Important
I used two disc-shaped neodymium magnets like these on Amazon.
The location of the two magnet switches can be located by sliding a magnet (or paperclip) across the frame of your mini. They are between the home button and the right edge. They are approximately 1 1/2 inches apart.
IMPORTANT: Your magnets need to be positioned with opposite polarity. I didn't have a compass handy to see which side was north and which was south so I just labeled them A and B. It also didn't seem to matter which polarity they had as long as they were opposite to each other.
Put a card over the area to the right of the home button and slide your magnets around until you successfully turn off your display. Try it a few times until you get a good general idea of the spacing.
Tape your magnets to the card to lock in their relative spacing and make sure one of them doesn't flip over.
Now slide your card around some more to make sure you have just the right spot. Since magnets are magical you may need to hold an amulet in your mouth while you do it and chant something. Seriously, it can seem like the target is moving, but be patient and you'll find the right spot.
Step 3: Attach the Magnets to Your Case or Cover
There are also at least two magnets along the right edge that you can use to keep your cover closed. Those aren't hard to find, and you don't have to be so precise. So add magnets to your heart's content. Just don't mess with the area between home button and right edge.