What’s the easiest way to make plumb dentil trim for a gable end?    …as pictured here:

https://www.finehomebuilding.com/2005/05/01/dentil-molding-on-a-gable-plumb-or-square

I would ideally like to use cellular pvc for ease of maintenance.  There’s lots of prefabricated trim where it would be square to the rake but nothing made for keeping the dentil blocks plumb on a gable application.

I have an early 1980s tract house that is a cheap rendition of a colonial.  I’m hoping to make it look more authentic by adding some more ornate cornice trim.