Will said in
post #5 of the thread you mentioned that since 3*AA alkalines can't produce the kind of current that would be dangerous to a P7, the light is basically self regulating. As far as the SST-90 goes, they require a lot more power than that kind of battery configuration can provide. Two problems are first 4*AA NiMH will produce too much power for a P7 without the addition of a buck regulator. And second the 2D Mag needs to be cut down and bored to accept a 4*AA battery holder. The machining, the battery holder and the regulator collectively represent a significant increase in the expense of your light.
Since you already have the 2D Mag, the least expensive option might be
3*NiMH SubC's. Three of that type battery is about 1/2" longer than a pair of D cells, so you need to modify the tailcap spring. You also need a piece of 1" schedule 40 PVC pipe about 5" long to act as a spacer. You can get that at any hardware store. And since SubC's are usually flat tops, you either need to stretch the spring in the switch body to make contact with the battery, or put a magnet on the tip of the battery next to the switch. Usually magnets are a bad idea. But in this case, the magnet is basically trapped inside the switch housing. It can't go anywhere if you accidentally drop the light.
If you want to direct drive the light, make sure you get a P7 with a J voltage bin. For a P7 bin code, when you see the letters DSXOJ or something like that, the J at the end refers to its voltage. J has a range of 3.5-3.75 volts.
BTW, :welcome: and happy modding