I don't have access to a work bench vise so I can make good square bends so I had a steel Fab and trailer frame repair shop I do business with make a couple of brackets for me out of 1 x 1/8 flat bar. After drilling screw holes through the tabs at the end of each bracket, I used self tapping sheet metal screws to attach them to the bottom of the trailer I-beam flanges.
I'm going to add a second hose carrier so I will probably drill two matching holes on opposite sides of the square fence post tube on each end lining up with the i-beams and use self tapping screws with fender washers at the inside top of the vinyl fence post to hold it to the i-beams. The holes on the bottom of the square fence post tube will allow me to get an extension bit through so I can put the screws into the I-beam flange with a drill driver. If I go that route I'll probably remove the brackets from the other hose carrier and reattach it with fender washers.