Well, if no flex j is the order of the day i can only see 2 viable options:
1. Order a completely new window with a built in J channel track/brickmold option then you dont have a thing to worry about
2. Custom make some trim/brickmold out of Azek and leave a channel for the siding to slide in behind.
I've made more than one trip out of town to pick up materials not commonly stocked locally. It'd be worth a few phone calls to find a vendor that carries it so you could at least price it, along with windsheild time to grab it, and give the HO the option. Driving a few hours never scared me since i get paid regardless:thumbsup:azek isn't av. in Cheyenne and i already tried ton talk him in to new windows it was a no go
push them on the idea of the pvc or at least make them understand that a new window is the only way with it j'd out or can you take the existing window and cut out the section of plastic which would otherwise be open for siding.
and flash it with 3m flex flash, same idea as vycor or bluskin only its designed specifically to work in arches