Step 1: Materials
1 belt of your choice
2 sets of any small nuts/bolts/washer (I used 3mm diameter threads) - these will attach the belt to the keg
1 set of larger nut/bolt/washer (I used 6mm diameter thread) - this will clamp the belt
3 large zip ties
1 large rubber band or old tire tube
1 angle grinder
1 dremel-like tool for finishing (you can do everything with a dremel, but it will take loooonger)
Step 2: Making
Empty your keg. You might want to do this one day before, as you will be using power tools and want to keep your fingers.
Take all the pressure off the keg and punch a hole or make a cut on the top (where the dispenser is) with your weapon of choice. Remember that grinding will add heat and pressure before you puncture the keg.
Get the green plastic ring off. I achieved minimal damage to the keg inserting many zip ties between the ring and the keg with the help of a screwdriver, until I could pull the ring off.
Use an angle grinder (or a dremel) to grind the inside of the top lid (see picture). Try to keep the grinding disc almost parallel to the top lid for best results. Pop the lid off and scrap the magic parts inside (don't tell anyone what's there).
As the logo will face forward, drill 2 holes on the top ring of the keg for the small screws (make them about 1cm apart). Grind a slot for the belt right below the holes.
Drill 1 hole at the bottom back of the keg, wide enough for your zip tie. I also recommend to grind a drain slot to avoid accumulation of water, right next to the hole (picture).
Cut a piece of the belt of (at least) 25cm. Required size may vary according to your bike.
Split the belt in two ends starting from the midpoint.
Punch (or drill) holes on each one of the 3 ends. On the wider end, punch another hole about 4 to 5 centimeters apart from the first hole.
Slide the belt into the upper slot on the keg. Mark where you drilled holes on the keg for the small screws. Punch 2 holes on the belt for them. In my bike, those 2 holes are 5,5cm away from the second hole on the wider side of the belt.
Attach the belt to the keg with 2 sets of the smaller bolts/nuts/washers.
Now the tough part: attaching the keg to the bike.
Step 3: Attaching
Warning: this is the tricky part. Also, you may want to adapt your project with some protective bits if you want to keep your bike scratch free (I haven't had such concern in my mind).
The pictures and your observation will help you more on this step than a description.
The important things I have observed from doing all this twice are:
1 - you want to position the keg in a manner that, after attaching the belt, the keg is held with its bottom lid almost facing forward (a secret zip tie will help you after you finished placing the large bolt on the belt - see pictures).
2 - after attaching the belt to the bike, you will force the keg down, gently crumbling the "shoulder" of the keg under the handlebar support.
3 - the bottom zip tie, held by an old tire tube piece, will force the keg to stay in place.
4 - It is perfectly possible to have a stable keg-basket this way (with no bottom support, only a bottom pulling force). I have about 50 km of fully loaded ride on my first keg, and it is still pretty tightly held in place.
5 - On my first attempt I added a piece of a tire above the fork to protect the frame against the keg possibly hitting its bottom against it. As it did not happen, I'm not using this protection on my second bike (the black).
I did this on a rush to spread the joy, so I probably won't be able to make any feedback-related adjustments soon. I'll be happy to receive any comments or observations tough.