"-MIKE-" wrote in message news
https://photos.app.goo.gl/DqXrs5DI2roJTli22

I need to replace the white board above this garage door. The 2x12 is
rotted out pretty bad. It will just fall out when poked with a
screwdriver or putty knife, so I'm going to replace with a new, treated
2x12. (Client wants cheapest option.)

...

Any ideas?

To start I would tack the new 2x12 (or 1x12 if it turns out to be that) up
on the side jambs even with the bottom edge of the existing arch board, or
level, which ever seems better based on the field conditions. Then I'd
scribe the curve onto the new board by following the existing curve. This
could be done with a large pencil compass. Alternatively a board that is as
long as the distance from the bottom of the curved board to the maximum
height of the curve and a pencil could be substituted. In either case make
sure the scriber is plumb as you move across the length of the board. Any
roughness in the scribed line can be smoothed out during and after cutting
the board... a slight undercut would help with the fitting.

Another consideration might be to get some 1x12 pvc trim (or glue up
narrower boards). If needed shim it out with with the scrapes from cutting
the curve. It would hold up better than wood.