If you want to go the easy route, you can go to Radio Shack and buy a KPR118 bulb. Then get 4 CR123a batteries and drop them in some sort of hose or pvc pipe so they fit inside the Mag. You will have to play around with the tailcap and spring a bit to make the negative contact work.
If you want to go the super easy route, buy a couple 3AA-1D adapters from Kaidomain. Buy a KPR112 bulb from Radio Shack. Put the bulb in and drop the batteries in.
Both of these are very cheap. $1.50 for the bulb, $3.00 for 2 adapters from Kai, then just the cost of the batteries.
Here's some pics: (note that the hotspot in these pictures looks brighter then it actually is. It's helpful to compare them though.)
Normal Maglite.
6AA's in a 2D with KPR112 bulb:
4 CR123s in a 2D Mag with KPR118
The Mag using the 4 CR123s is obviously brighter. However, I like using the AAs with the adapters. You can use normal alkalines or rechargeables. While it's not quite as bright, you don't have to mess with the Lithiums and worry about the rare chance of explosion, fire, etc. It's also cheaper to run if you use rechargeables, vs $5.00 everytime you have to refill the tube with lithiums.
If you want to do more, you could put together a cheap ROP LOLA.
2D mag.
Borofloat lens (about $5.00),
Pack of ROP bulbs (about $8.00)
Kaidomain Aluminum reflector ($15.00)
6 Eneloops AAs ($12 to $16?)
2 Kaidomain 3AA-1D adapters ($3)
You would end up with this:
Then you could brighten it up more with the resistance fixes, Deoxit, etc.
