I was nervous about that too but so far there has been no issues trailering the mast in 1-piece.  Since the mast is oval'ish, I do ensure that the longer cross-section is vertical, because that will be stronger.  
To secure the mast to the aft crossbar, I bought the thickest pool noodle I could find at Walmart, which turns out to be shaped like a star and sits pretty well on the crossbar.  I cut about 12" of noodle and use two tiny bungie cords to stop it falling off while I position the mast on it.  Then I use two of those black rubber type bungie cords and wrap each around the mast and cross-bar, hooking them to the tramp laces.  I use two for added security.  If the noodle starts to look too squished after a number of trips, I just cut a new 12" section.  It seems to last pretty well.
I'm sure a PFD would work too.
I would take a picture for you, but I'm stuck on the couch with an injured foot 
