Let me start by saying I'm not in possession of any Yakima product, just a gadgeteer.. From looking at the 3 pictures provided, where it me I would get another mount, OR, more likely, go to a hobby shop and pick up a small block of Balsa and some coarse sand paper. Since I am a long time Modeller, I know how to work Balsa, and it is an amazing wood. Next wrap the sand paper around the cross bar, and run the balsa block back and forth until you have the shape of the cross bar. Now take the block and using a pen mark the top part of the clamp. I think you can see where I am going with this.. you are creating a thin shim, slightly oversize it so that the balsa will provide enough wood to supply a tight fit. Once this has been accomplished the last step is to use Lacquer, or AeroGloss polybutylite (Model airplane dope), any color will do.. but I would thin it with some clear thinner. What this does to the balsa is soak into all the wood pores, and become very hard. Another option I have used is to mix epoxy and spread it over the newly created balsa piece. Either method will produce a very hard yet somewhat resilient piece.
OR, just go back to the store and buy the next bigger size...