Preferably the front light will pivot within a low-profile enclosure rather than swinging on an arm, and the kit will have a rear-mounted battery pack with a bright red rear flasher, a high-power white front light, a stretchy coiled wire connecting the parts, and take relatively high capacity AA batteries. LEDs are a virtual necessity for a long, bright life. An old one with a dirty or bad strap would be perfect, since you won’t use that part anyway.
Other adhesives may work, but finding one that will bond to but not attack helmet materials such as a mix of plastic and foam can be tricky. Clean all mating surfaces and allow them to dry for a good bond.
Placement of the battery pack is flexible if it does not have a rear light, but the rear light, whether or not integrated into the battery holder, must shine straight back. The battery door must be openable when the holder is fixed in position. When test-positioning components and checking illumination angles, tilt the helmet to face straight forward as it would be in use, rather than tilting down as it would sit on a table. If the headlamp has separate front and rear components connected by a cable, the position of the front light forward to back along the crown may be chosen to take up excess slack in the cable. (The cable should not be kept in tension to avoid stress to it, to avoid stress to the components, to avoid stress to flexible adhesive such as tape that it may pull loose and crooked, and because tolerances help ensure successful construction. )
If it has aerodynamic-looking tail ends of spikes at the back protruding well beyond its overall thickness, you might carefully trim these in a relatively sheltered area (not the rear extremity of the overall helmet) to create a small flat mounting area while not harming the overall structure. Epoxy is very strong and headlamp components are very light: a large mounting area is not needed.
