A grey flag, then?
A grey flag? I'm not sure that taking a close-up of the grey card held in front of the lens at arm's length would provide an accurate color balance. Doesn't it have to be placed in the scene somewhere?
The problem with the Sigma 15-30 is that the front element of the lens protrudes greatly. It has great curvature. It's almost as curved as a tungsten filament light bulb. In order to avoid vignetting in the corners at 15mm with FF sensors (or 35mm film) the fixed lens hood is particularly short with a sort of serrated edge.
I usually try to place my hand between the direct rays of the sun and the lens, but some sort of card would be better. I'm annoyed that the following shot shows a finger tip in the top left corner .