You could also use a butt block. Pigmey and Glen-l both use butt blocks in many of their kits. You would use a scrap of plywood with the grain running parallel to the side panels and glue it on the inside. You would be surprised how strong it is and how fair it will bend. I should be about 6" wide. You could leave a space on the top edge where Chris calls for that shear strip he uses.

This has been an ongoing debate between Chris and Pigmay which way is best etc... If you have the length you may go for the scarf if you don't I wouldn't hesatate to do a butblock.