The use of an LED retrofit would certainly greatly extend the run time, but a cost of $10/£6 would no longer be a throwaway item.

I also suspect that an LED retrofit bulb would be destroyed by the heat of soldering wires to it.

For my own use, not for giveaways I have some lights that use 4D cells and a miniture screw base bulb holder.
These use either a 5 volt 0.5 amp incandescent bulb for a run time of 20/30 hours or an screwbase LED lamp for 100+ hours.