I love how people take two chunks of aluminum and file them to make an everlasting gooseneck. I am not able to find thick enough aluminum, though. I saw Chuck Millican's gooseneck at Champions. A little easier. Plus the long aluminum channel (for a lifetime+ of rigs) is in the metal bin at Home Depot for $11.
Chuck used the larger mouth channel for both the boom and vang, as shown in my second post.
The idea is to cut a couple of openings on the aft side of the mast. A tight fit is best to prevent unwanted mast bend at the gooseneck. Put a piece of C channel in place held there with a screw/bolt through from the front of the mast, Bolt a Dubro swivel ball link in place on each one similar to a standard gooseneck.
The rest of it can be done using methods shown on the EC12 building page,
http://ec12.info/MY/MY168Vernon.htm
Cutting a Dubro 2157 turnbuckle should allow you to cut RH and LH threads in plastic Dubro swivel ball links as needed. The slight burr allows us to cut threads in plastic just like a real thread cutter..
Finished bits shown in second post.
Scott