Depends on the profile/cross section of the trim, what type of plastic,
and if it is pure plastic or half sawdust, like some. The stop molding
I recently put on the garage was like a cooked noodle, so flexible it
was hard to put up. Now if you are planning 1x4 cross section, it may be
a little harder. Go to lumberyard, and play with the stuff in the rack.
If it is too stiff, you can do the lumber trick of slotting the
backside, and fill the visible cracks with putty or caulk, or cover them
with some other trim that will bend as needed. Worst case scenario, if
the window in question is still in production, the vendor may have a
purpose-built trim kit available. That is likely the expensive option,
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