The German soldier was much too disciplined to start modifying their equipment on a whim. The holster was issued to protect the pistol, and getting off a fast draw was not a consideration. Sounds like you need a P.38 if you want to use it's holster.
The Germans even went so far as to stamp the holsters with the pistol it was made for. A Luger holster won't be much more expensive than a P.38 holster, and it is correct.