I'm not sure why you think the 1.2K is an error, a dip right after the baffle diffraction peak is quite normal in sharp edged cabinets.
I'm sorry if i wasn't clear, but i wasn't referring to the 1.2kHz dip, but instead, to the 1.2kHz peak.
RED: Far field, 1.7m from floor; Green: Ground Plane; Purple: Nearfield with diffraction:
You might need to do some experimentation to see where the best place is to get the longest gate.
I've measured the attic, and the exact (rounded) measures are 7920 mm x 5800 mm.
So if i use a circumference with a 2900 radius, and given the speaker is 1000 x 400 x 400 mm, it would give me 2.7m free of reflections:
If you want to avoid using the platform then experiment some more with the ground plane placement. You can look to try and undo the extra baffle added by the ground plane by comparing the difference in Vituix and seeing if it is significant enough to worry about accounting for.
I'll look into it, thank you for your detailed post!