Upon removing and replacing your hot mirror, I think you'll need to manual focus to compensate the focussing...Since your camera focussing is calibrate to normal light focussing. IR has shorter wavelength than normal light.
There are stories of people using microscope slides as the clear glass. You might want to try. But 3.09 mm is darn thick. Most slides are about 1-2 mm. You might have to stack them to get that thickness.