I have one too!
I have one of these 4x8 Harbor Freight trailers that I have it set up to haul two kayaks. I have used 2x4 lumber to build racks to elevate the boats just about 8" above the flat bed. I have mounted my Malone Kayak Saddles on the 2X4’s. All of my fastners on the trailer have nylock nuts and I have pulled the trailer ove 70 mph. I have not had any problems. I have about $215 invested in my trailer and wood. Thanks