This is my card template and the measurements and scorelines.
I decided to use a bright fuschia card and to adhere my post-it (one pad halved - it makes them go further!) on the turned-over card as shown:
I rounded the corners and decorated the front of the holder so when folded up it looks like this:
and unfolded it looks like this on the inside:
The black square is the "hooky" portion of some velcro tape and the other half of the tape is on the front of the note-pad section. You could equally use a sticky-on magnet.
So there you go! Subsequent wallets should not take me two hours to make!
Ooh it's getting a tad gloomy around here now; I feel some eye strain coming on...