Along the lines of what Jim said. Buy a length of 3/4 dowel rod at the hardware store. Cut a straight piece for a template that is 3/4" wide, same as the groove. Cut a slice off the dowel rod the same thickness and then cut it in half and glue that half to the end of the straight piece for your template. Now you can clamp it to your runners and use a flush trim or pattern bit to round the ends. Nip the corners off the end of the runners first and that will make it easier to rout.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.