Weld or bolt eye bolts to the ends of the cross members and to the center upright where each cross member crosses.
I built this kayak hauler for a friend. I spent a good bit of time thinking about the hitch. My favorite part is using a short piece of automotive heater hose.
Kayak Trailer - Harbor Freight Mini Trailer
(Optional) If you need to further stiffen the cross-trees, weld a pair of small struts from the center cross members to the ridge member as shown in the diagram. The separation of the lower struts is exaggerated for clarity, they should connect only