I would think a slight camber is going to be the ticket. Square corners would work if you were just joining edges on boards, but in this case it will leave a ridged somewhere. Rounded corners I don't do, but suspect they would leave some sort of mark as well.

A wider blade along with a shallow camber should do a reasonable job. The wider the blade the more you will be able to hide the camber. When sharpening just work the sides a little more.