I would most likely just make a piece of corner molding out of the same type wood as the bead board. If you wre to look at the piece from the end it would look like an "L" with both legs the same length. You can sometims find this at home centers or build supply places but my experience is that it is to small d to do the job so just getting a piece of lumber of the proper spieces and cutting it to size, outside dimemsions, using a router to round the outside corner and then cutting out the inside part of the "L" on a table saw.

Hope this helps.