Here's my homemade ultralite light:
It weighs 0.3 oz, and is bright enough to read by and illuminate my tent or walk with.
The battery is a 3.6V lithium cell, 1.1 Amp-Hr 1/2 AA.
The LED is a 3.6V ultrabright white LED. It has been carefully squished so that the leads touch the right part of the battery at the right time.
The "case" used to be on the end of a tube of chapstick, and was precision machined with a soldering iron.
The switch is a slide switch: Slide the case forward for off, or back against the battery for on. You can see on the side of the battery the plastic covering was melted to provide a contact point.
It lasts about 15 - 20 hrs per battery, and the voltage droop characteristics of the lithium cell is pretty flat.
Total cost was about $4.