Y bracing existing deck

HI all:

  I have an existing deck supported by 6x6 posts that are 8' tall. The posts support a triple 2x6 beam.  I want to install Y bracing. For aesthetic reasons I wanted to install 6x6 Y braces on the inside of the beams but worry there is no way to fasten this to the non-solid triple 2x6 beam.  Do i have to attach to the outside instead using say, 2x6?  

http://imgur.com/gDFxqFP

Thanks,

Holdfast4