I have always carried a meter key on my keys and it does get used a few times a week at least. More recently I found the need to have a cabinet key on me often and all the ones that you can buy are bulky plastic or big metal. Nothing small or a combination of the 2.
Step 1: The Mod
So I got creative. I got my well tarnished metal meter key and added a slot and trimmed the sides. It was easy to match the shape up. No need for a round centre piece as the hole is already cut in the key. I never understood why they put 2 key holes in these things anyway!
And that's pretty much it. They should exist anyway.. but as not I made my own. As I always do with these things.