You may want to try bullnose trim?
I think they sell it at Home Depot or Lowes. But it would be the holy crapload of work to finish it off nicely.
route, glue, putty, sand, putty, sand, prime, sand, paint........ugh
And you would also have to trim off the top gap created from the bullnose.
On second though the sheetmetal flashing as mentioned above may be easier.