I have a few 10mm FET drivers from Mtn, 10mm is really small, he used really small components to make it fit and there is no way that a normal size 7135 can be added to make it FET+7135, I’m not surprised that it is a puzzle to make this driver.
A good solution IMO will be to omit the 7135 and make it a basic FET driver. The 7135 on theGT-mini and Micro drivers is to make the low modes more efficient because then you do not need PWM-ing the inefficient direct drive current. But the direct drive current in 10440 flashlights is not extremely high anyway so efficiency is not bad to begin with. So there is no need for a 7135 in this battery size and I’m sure that without it a fitting driver can be made by Lumintop. Hint!