I'm sure a flat piece would work. I can calibrate the meter to whatever I choose to use. I'm just not sure what I want. I found the Sekonic user's guide online, and it says
Use Lumisphare (sic) when you wish to measure incidend light for portraits and 3-dimensional subjects...point the Lumisphare accurately toward the camera...
Use Lumidisc when you wish to measure lighting contrast and lux as well as exposure of single-dimensional objects...point the Lumidisc accurately toward the light source...
I'm not sure what a single-dimensional object is and why you would be photographing it.
To be honest I'm not sure how one uses an incident meter. I always figured you held it in the light you were interested in, I just am not sure what direction you are supposed to point the 'dome'. If I'm outside in the daytime shooting 'street', I would point the dome straight up toward the sky I suppose. But if I have someone sitting in a chair, side-lit by a window, do I point the dome at the window, or back toward the camera?
It makes sense to me that a flat piece is going to be significantly more directional than a hemisphere would. I don't know if that's a good or bad thing.