I was at Academy Sports the other day and found these relatively inexpensive ($114) guide-on bunks for my trailer: http://www.academy.com/shop/pdp/ce-smith-company-carpeted-bunk-board-style-guide-ons-2-pack?repChildCatid=13236 They are a big help in a cross-wind and the boat always slips into place in the center of the keel trough (or keel rollers in my case).
I mounted the support stanchions on top of the trailer frame to get the bunks up as high as possible.