Just need to find a driver which can produce the current you need which will allow the amount of input voltage six Li-ion cells can produce in series; 6 x 4.20 VDC and then heat sink it well. I've been searching for one of those for quite awhile and have not found one designed for laser diodes, not at that high of an input voltage. The squeeze on finding one is this; there aren't very many laser diode drivers which are high current as well as high input voltage, I bet they are out there, but I haven't found one I want to pay for yet. Even if cost isn't a factor, I still am having a hard time finding one exactly what I need for a VIS laser diode. I can find them for IR, no problem, not blue or green yet. You might be able to find a high current CC (constant current) driver made for other use and see if it spikes when turned on and off and also test to see if it has a soft enough start to baby your diodes, but that might require just buying something and trying it out on a dummy resistor to see how it acts with an oscilloscope hooked up to its output.
Why you want to run six batteries in series? Is it because you need the higher voltage to run the diodes in series? If not, don't do it because finding a CC driver designed for laser diodes with that high of an input voltage at the needed current might not happen, if you can, please tell me! I know one is out there somewhere, but am still looking.
If you want to use six batteries just because you found a nice host which uses six of them, you can make some dummy batteries out of solid metal, insulated from the tube of course, to fill in the tube so you use fewer batteries and thus less voltage.
Do you want six batteries because you think it will extend the run time? It can some, but paralleled batteries will extend your run time more efficiently than wasting the extra voltage produced by using too many batteries, or rather, having far more voltage than you really need which if using a buck driver must be dissipated as heat. Only issue there is to make sure you parallel CHARGED batteries and batteries of the same type! Not one drained battery and one charged one, that could quickly cause an overheat problem and explosion. This is something you will need to study up on, paralleling Li-ion batteries, before attempting something like that. The TK 75 flashlight does it, but they also give the warning not to parallel a discharged battery with a charged one.
If you want to use six batteries in series, look for a switching CC regulated driver board, that way the extra voltage is not lost as wasted heat, but finding one suitable for laser diodes might not be easy. If you do, please let me know, I want one which will operate up to 6 VDC of output at high current too. Here's a switching voltage regulator you might consider putting in line between the batteries and a normal laser diode driver but it won't take 25 VDC, only 14 VDC. I have an idea though, since they are so efficient, you could run two of them with their inputs in series with one another and then it will take 28 VDC input, as long as there isn't a common ground problem. Maybe Texas Instruments or someone makes one which will work without needing to do that?
http://www.ebay.com/itm/Texas-Instr...o-5-5v-16A-Dip-Module-Qty-1-NEW-/191947868446
If you use two of these with their inputs in series so they can run off of 6 Li-ion batteries producing about 25 VDC, you will then have two separate 5.5 VDC outputs which can pull up to 16 amps each. Is that enough voltage to run some normal laser drivers for the voltage required for your diode? Since the output is only 5.5 VDC maximum, the drop out voltage of your laser diode driver becomes very important if you need 5 VDC. Off hand, I don't know what the required voltage range is for the NUBM44 diode. (Edit: under 5 VDC).
If there is a common ground problem using two of these voltage regulators with their inputs wired together in series, you might be able to get around it by insulating them from one another and or the host. They are fairly small, so if using a flashlight as a host you should have plenty of room for them. An advantage of using them is the current draw from your batteries will be fairly low compared to the current they can produce for the diodes.