I have an 8' length of 6" pvc secured on top of our bumper with three heavy plastic zip ties. It sits on three isolation pads to to avoid wear an corrosion between it and the bumper. There is a 6" cap on one end and plug in the other held in with a 1/4" wins screw threaded in to the tube. Have unbelievable number of items stored in there. It's been there for about 13k miles.
2018 Eagle 321RSTS
2017 Silverado 2500D, CC,LB,SRW
1973 IH 1210 CC
1975 IH Travelall
1979 IH Scout II