I have a set of Seawings on the trailer that I use to transport our Shearwater Double, which has a 27" max beam.  If the SWD were any wider where it rests on the Seawings, it would be too wide.  With a beam that is 6" more than the SWD, I am not confinced that a MC 16.5 will fit.  Measure the width of the boat at the two loactions where it rests on the bar.  If it is over 25" then it is too wide.  As an alternative, I would suggest a set of saddles which is what we use when we car top the SWD.  Saddles on the front provide excellent security and a pair of HullyRollers on the back makes it much easier to load.

   OK the Seawing is out of contention. I do have a Thule Slipstream (which I think is now discontinued) and it has a pair of brackets that I think will work well. Don't know the Thule name for them but IIRC they came with the Slipstream and are the type that can be adjusted to different angles to match the angle of the hull. I should look in my garage more often - saves me a lot of money when I dig around in there - find things I'd forgotten about - 

   I find that the width and relatively flat bottom on the MV16.5 means that it travels best flat on the bars. I use kayak saddles on wither side of the boat on each bar. They are adjustable for angle so you can get a very snug fit. It is quite quick to load? and once cinched down you get zero movement.

   Sorry for the empty message above. I'm pretty sure that Stax Pro2(review is here) is preferable to Seawings. Up to 2 kayaks(50 lbs+ is not a problem), lightweight and durable and the price is less than $100 atm - what else is needed?