I mounted mine on the top of the bumper. Mounting it on the bottom looks like a disaster waiting to happen when you pull out of a gas station with a significant dip in the pavement.
I also used what they call an end post as it has 3 rectangular holes punched in it for the fence boards. They provide enough ventilation to help things dry out inside the post. I mounted on top of the bumper with 1" spacers and the slots facing down. I used large air conditioner duct zip ties to hold it in place.
Holds 2 pieces of plastic gutter and a 20' hose with all the connections attached. I suppose it could hold more hose if needed.