Go to a timber yard (not B&Q) and buy MR-MDF bull nose window board from them, it comes in various widths. Cut to length for the window recess including allowances for the "ears" at each end. Cut to the required depth including allowance for front overhang. Work a bull nose edge roughly at each end with the plane or spokeshave. This takes far less time than using a router (in point of fact it takes less time than getting a router out of its' box). Sand to finish required (MDF is very soft and P100 sandpaper will take out minor facets left from the plane easily). Cut the notches on either side, forming the "ears". Fit. Why so much resistance to a bit of basic hand woodworking? This is what a working carpenter would do
Using a router is slow, cumbersome and error prone in this instance for what is an extremely quick, simple and easy task to complete with a sharp hand tool and some sand paper. In addition you will be left with a large flat (approx 8 to 9mm) in the middle of the edge (necessary to accommodate the follow bearing on the cutter) if you use follow bearing cutters - the only option with a 1/4in router