I think you’ll find it challenging to drive an XHP50.2 or XHP70.2 in that light hard enough to beat what you could do with an SST-40 or Luxeon V driven with a FET or one of Led4Power’s linear drivers. I guess a 2x18350 config might be able to do it. A 17mm MTN-BST2 might be able to, but they are out of stock and cost, on their own, as much as a complete 3v pocket-rocket would.