I think your customer has experience.
Those sides either get rolled or folded a lot.
Even a perfectly made banner (vinyl/vinyl) will look like crap after being rolled and folded
a few dozen times.
For the purposes of flexibility (and non-cracking) I'd be thinking a water-based
sign paint on
those sides once the surface was properly cleaned and prepped.
I am far from an expert at that so I'll be watching the replies in this thread.