I used carriage bolts up through the 2x4 into bottom of the PVC and then secured with a flange nut. I used U-bolts to secure the 2x4s to the trailer cross-bars. I give lots of credit for the idea from seeing how Tom Kirkman rigged his trailer.
Tom's suggestion to this post is also valid. I could as he suggested dump the trailer crossbar and mount directly to the trailer frame. To do that I'd have to work around the spare tire which is doable. I'm not sure I'll go that way because with the crossbar, I can still carry another yak using J-craddle and I kinda like having the boat a little higher on the trailer. It allows me to see it well through rear view going down the road. His point is the advantage of launching in the water and that's certainly true.
Thanks for the comments