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:
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.