I would epoxy some aluminum strips on the back side of that lip
for strength, then bondo and smooth over the cracks,......or strengthen it with aluminum and evenly spaced pop rivets for a race car-type look and smooth it, etc...

On my car I made a spoiler from inexpensive soft material, so it flexes
when needed, and even will wear away instead of being rigid and/or cracking/breaking when it touches the pavement.

Catch me at some of the meets which I go to. Maybe when we look at it, we will find better solutions....if you don't want to buy another one.-Bob