If you could find two parabolic mirrors with a focal length difference of less than a couple of inches, I suspect you could take the middle of the short focus one and the outside of the longer focus one and stack them concentrically, to get a flatter dish.

Honestly for this use case, just sliced and stacked a single dish would give you mostly-collimated light, right? If you put this behind a mild diffuser you could probably still trick your brain unto thinking it's sunlight coming through the curtains.