Just use 2 screws (I used 16mm M3) + 4 washers + 2 nuts to secure your keys.
I'm sorry the pictures are not more descriptive but I have an aversion against posting pictures of my keys on the internet. :)
This thing was made with Tinkercad. Edit it online https://tinkercad.com/things/7FiWYLp03HY
Update: I made a shorter version of this thing (V2). You can download the STL here / or edit it at Tinkercad: https://tinkercad.com/things/kVRWqtAy9Dn
Update 2: I updatet the STL (now V3) for use with 5mm diameter book binding screws. I also uploaded a version with a separate keyring loop.