I'm still thinking of doing a hollow mast. I made up a quick jig on an 18v hand saw to cut the groove in a single batten, then I cut this into 8 pieces.
Even with this rough and ready attempt, it came out rather well. Go to Duckworks website and search for Birdsmouth. They have LOTS.
I penciled in the outline of the joints, othersiwe you could hardly see them.