I offer to the group some details on my trailer setup. I have a Hobie (Trailex) Double Double with tower and storage box. Currently I haul two Outfitters but hope to replace one with a Tandem Island soon. I secure the Outfitters upside down on the trailer crossbars.
The kayaks sit on cut sections of a horsepen mat I no longer needed. Primary securement is via normal kayak straps over the hulls. But note the straps go around the crossbars, not just the strap mount itself.
To prevent movement aft I use a pair of the Hobie multi straps attached to hooks I picked up at Northern Tool.
To prevent forward movement and provide security I use a locking cable attached under the main trailer spine.
It could be argued the hull straps are sufficient but with the other devices I do not need to make the hull straps very tight so no risk of deforming the hulls.
When I bought the trailer and the tower, there were plenty of extra lock bolts, nuts, and washers to attach everything else including the tower box.
The box is a SportRack SR7018 and easily swalows 5 life jackets, 4 Mirage drives, 4 paddles (diassembled), and everything else we take except the sail kits.
I have enjoyed several trips and covered several thousand miles with this setup without issue.
Peter