I found a similar fix, though my stern rail is not as curved.
I also went to WM and bought the small, white, two-part panel but was told that WM no longer carries the 'flag pole, split bracket. Jeff, the WM staffer in S. Daytona, thought about it and came up with a Bimini top mount bracket which hinges at the bottom. (See Web Page Picture)
I took the 1" ss tube to the local machine shop and had them cut out the tubing so it would fit flush with the bracket shoulder. They also to drilled two holes in the tube to accept the ss screws that hold the bracket closed once mounted on the rail. Put the bracket on the bottom rail, close, then put the straight flanges inside the tube, put in screws, tighten, install tube into motor bracket and then the whole assembly to top rail. Make sure you buy the correct size for your rail.
Here's the 7/8" bracket page:
http://www.westmarine.com/webapp/wc...Primary Search/mode matchallpartial/0/0?N=377 710&Ne=0&Ntt=Bimini_clamp&Ntk=Primary Search&Ntx=mode matchallpartial&Nao=0&Ns=0&keyword=Bimini_clamp&isLTokenURL=true&storeNum=18&subdeptNum=58&classNum=595
Mine looks the same as BobR's pic

except that the flanges fit inside the ss tubing.