A tall plant beside it? Or a vined plant on top to hang over and hide the off center look. If custom made, can you ask the carpenter to add to the side of the cover (would have to be cut around the pipe but could be done).

Give it a week or two and see if it is still bugging you then.

I think you have to divert the eye and if you added Roman blinds and a chair on the right on an angle, it should help.

I would have had it built to go around the pipe also. Wide enough that it covers the same amount of window on the left as the right Is that a water pipe? It may also get hot.

You have an off-center radiator, so what did you think would happen when an enclosure was built for it? It's a nice looking cover, and unless you told the person who built it that you wanted it bigger to be symmetrical, it's a logical outcome. If it were me, I wouldn't want it so big that it was symmetrical and blocking the lower part of both windows. Unless you want to re-pipe and move the radiator, I think you have the best minimalist solution.