If you leave your mast attached with one bolt at the tabernacle (hinge), the top part of the mast will stick out way behind the boat, causing a danger to others and more significantly, place undue stress on the mast. What most people do, is make a stirrup on an extension of the winch post of the trailer and cradle the base of the mast there. You can use a scissor crutch for the back, or construct something that fits into the rudder gudgeons to support the back (top) part of the mast during transportation. Check pages 1 and 2 in my website album to see how this is done.
http://groups.msn.com/RogerConrad/shoeb ... lbumlist=2
Don't forget to tie the mast down during transportation. Also if you are travelling a long way, you may want to secure the middle of the mast to the tabernacle or support it with some cushions or old PFD's.
As for the mast plate, there is a backing plate under the cuddy floor and integral to it, supported by a short post that is fiberglassed in both top and bottom. The plate just screws into the cuddy floor and into the backing plate. You can see it on page 2 (4th picture) in the above mentioned website.
I have a book on maintenance and repair of the DS if you are interested. See the top note in the DS II section of the general forum.