Dnos:
We're trying to prevent a pizza from crumpling into the side of a box when the bike is tilted at up to a 45 deg angle.
We're also trying to make a horizon-locked platform on a bike because it's awesome and if we try, we can.
The net force is not orthogonal to the lean angle of a motorcycle. In theory, yes. In practice the rider is never leaning at the same angle as the bike.
Lateral forces (the ones you want to reduce) will be greater if you keep the pizza parallel to the horizon than in case where the pizza follows the leaning of the motorcycle.
You do not want to keep the pizza horizontal, unless the bike is stationary or cruising (but in that case there is no leaning).
I'd keep the roll axis fixed, but compensate for pitch, because it is breaking that produces uncompensated acceleration.