This is how I joined the corners on the large box frame, lots of Titebond wood glue, a 40mm aluminium dovetail key and then it was cross drilled and screwed. The problem with this joint is each screw is potentially weakening the timber as there is so little surface area to begin with, anyway I was happy that the joint was as strong as I could make it. It was also possible to put some screws through the batten surrounding the piece being framed, into the inside edge of the frame.
