Before anybody told me you couldn't do it, I built a standard boxmod with parts from Maplin that was a 4.8v AAA. All you need is to be able to build a basic circuit with an inline momentary switch in the +ve and a killswitch in the -ve. The hard bit is soldering in a 510 connector, but there are loads of guides online for that. You should always use rechargeable batteries as these are protected, and also have the bonus that generally an individual battery comes in at 1.2v, not the claimed 1.5, so using 4 in series will give 4.8v instead of 6. I should add that I don't have a clue if this is deemed as "safe" or not, I can only tell you that I use it as a spare and it hasn't killed me yet!