The simplest approach is to shoot in RAW and set white balance from a reference frame rather than relying on in-camera WB.

Good options:

• Photograph the light table by itself so it fills the frame.
• Even better, place a blank/clear transparency on the table and photograph that, since both the light table and the film base can introduce a color cast.

For the reference shot, expose so the frame isn’t blown out; stop down or otherwise reduce exposure until it records as a usable light tone rather than pure white. Then use that image in your RAW editor (such as Lightroom/Adobe Camera Raw) to sample white balance and apply the same setting to the rest of the images.

If you don’t have a blank transparency, use a clear area from one of the transparencies as the reference.

This gives more consistent and accurate results than trying to guess the white balance in-camera.