So you've got a fixie bike. That's cute. But if you're using a bike lock with the mother of all ugly keys, it won't do you any good in terms of street cred. It's attention to detail that will make you stand out of the crowd. Maybe.

So my bike just got stolen (they shall burn in hell) and I had to get a new one, including a Trelock bike lock. Since I have the key in my pocket I noticed that it is surprisingly sharp and threatening of punching a hole and ugly as hell. So, since I had two keys, I decided to open up one and see if I can make a nice cover. It wasn't really planned, 't was more a "key in hand and hey there lies a plier" thing. When I had pried it open I was extremely pleasantly surprised that the metal part had the exact shape I wanted. It's like they made it with this hack in mind. It's positively begging you to do it.

You will need:

5 minutes of your time
a scrap piece of leather (I used veg tan leather [1.5mm thickness] and Fiebings Oil Dye)
a chicago screw (aka. sex bolt, barrel nut)
a rivet with some setting tool
some basic tools [cutter, hole punch]
obviously a key of some kind

Step 1: Disassemble the Key

Should you use the same lock you can just apply some pressure with a side cutter or a screwdriver to the groove indicated by the arrows. Should you not be so lucky you might have to take off the plastic cap by force. And if the resulting piece of metal isn't shaped appropriately you might have to drill a hole and do some filing.

Step 2: Determine the Size

The key piece should stand out a bit on one side and on the other side there should be enough space left to attach a rivet or - alternatively - make a few stitches. Veg tan leather can be very easily folded when wet, so apply a bit of water to the fold. Cut into shape and make two corners roundish (cutting several straight cuts along a dime will do the trick, don't try and make a curved cut).

Step 3: Make It Meaningful

Since we're paying attention to detail here you can use some punch letters and give it some meaning (again, make the leather wet for it). The piece of leather was too short for a poem of Tolkien, so I wrote BIKE. Very handy should I be at some point too drunk to know which key it is.

Step 4: Punch Two Holes

Depending on your rivet size and the size of your Chicago Screw, punch two holes through both layers. The key should stand out a bit on one side and the rivet should have some space to the key.

Use a rivet setting tool of your choice and join the two sides together.

Should the stem of your Chicago Screw be too long for two layers of leather and your key, you can take a tiny piece of leather and make a washer. Alternatively you can use washers, but they might be a bit noisy when shaken.

Step 5: Assemble and Enjoy

Should you have used dyed leather you might want to apply some finish like acrylic resolene or bag kote to prevent it from coloring your trousers and oil/wax it so it withstands the years of use to come. Coloring and burnishing the edges is nice too.

Put everything together like shown in the pictures and use it with a confident nonchalance. It won't do to brag about such a tiny detail.