https://youtube.com/watch?v=3ozWlzmvsew&si=Lzu21dmHkdFXexur

i came across this YouTube channel, a NZ guy building a 7m trailer sailer with a Ljungstrom rig (of course it would be better junk rigged!)

His mast needs to be straight sided, so he joins two 4” tubes with an internal joining piece.  
He is making a beautiful job of the boat, well worth a look at the whole project if that is your thing