Keep it simple. Get a Light & Motion Urban or Deckhand light - same thing. That specific type of barrel is ideal for this homemade rig.
Besides being a good all around light the L&M has reasonably flat discharge characteristics, decent battery life per charge, and the older models had a pulsing rather than strobing type option. Opinions differ about which is better.
These lights can be strapped to most handlebars, head tubes, some forms, and road bike helmets without special accessories.
Then get an empty pill bottle made of white translucent plastic, usually HDPE. It's soft, easy to cut and trim with a craft knife, pocket knife or scissors. And it's tough and durable.
Cut the bottle to fit the notch in the light barrel behind the lens. It should snap in securely yet be detachable without damage.
Trim the shape of the hood as desired. Mine directs most light where it's needed without blinding oncoming cyclists or pedestrians. It can also be rotated to light the road ahead at longer range with less spill on the bike and road directly under the bike.
I used black Gorilla tape to shield my eyes, while leaving some white plastic visible on the sides to enhance side visibility. This is ideal for handlebar mounting but not necessarily the best for helmet mounting.
Besides enhancing side visibility the bottle hood creates a larger diameter apparent light source as seen from the front.
Completed assembly of hood on Light & Motion Urban 500.