I used a Dremel to get the job done here, but you can easily do the same thing with a pair of scissors. A hole punch would be handy as well.
All, right let's make this thing!
Step 1: The membership card
Here you can see the main card as well as the two smaller ones that you can put on your keychain.
Step 2: Notch the big card
Step 3: Notch the smaller cards
Step 4: Cut out the holder
Alternative: Use a hole punch to punch out the curved bottom of the notch and use a pair of scissors to cut out the rest.
Step 5: Put it all together
Step 6: Add iPhone
I originally planned on the phone resting on the big card, but the notches in the front held it on their own. It's still nice to have the full-size card in the back as you can use it to guide the phone into the notches.
That's it, now your iPhone can stand up for itself!