You can do what I did for the bike rack. Screw 2 eyebolts into the rubber trim that covers the corners. There is about a 1 x 1.5" alum corner behind it. I attach a rachet strap to them to stop bikes from bouncing.
Other way is to go under the frame to add brackets which will allow the box to stay on its own.
It was a rainy few days at Algonquin Prov park...tarps kept us dry.
Wayne --Belle River (Windsor), Ontario
2013 Sunset Trail Reserve 25RB..(just sold it)
2015 White Chev Silverado 2500HD 4x4 (6.0 l gasser)
2003 Mountain Star 890SBRX Truck Camper