Hmmm. Maybe I didn't describe the problem well enough.
The gun won't go any deeper into the holster due to the grommet/rivet in front of the trigger guard, so it's not really a problem with up-and-down movement of the pistol. The problem is that the slide is so short, the pistol can pivot around the front of the trigger guard. The longer slide of the G34/G35 extends deeper into the holster, preventing any rotational movement to the rear, as the front-sight area of the slide hits the front of the holster and acts as a brake.
If you stick a block of hard foam in the bottom of the holster, it will make the pistol sit higher, but by lifting the trigger guard farther off the grommet/rivet, it will make it easier to rotate to the rear, not more difficult. Unless you lift the pistol so far the the flap absolutely jams all rearward movement of the slide cover-plate area (difficult, if not impossible, due to the springiness of the flap latching mechanism; the flap will move up with the pistol), the higher the pistol rides in the holster body, the easier it will fall out the rear of the holster. The thumb-break does immobilize the cover-plate area of the slide, but I don't think the flap will, all by itself.