Here's a place for the 4 inch tube
https://www.mount-n-lock.com/all-products-category
You are likely better off finding a welder and have them fab up a bumper with brackets
Then it's what you want specifically.
If it was for myself and to store sewer hose.. You can either buy the plastic tube from camping store rv dealer and mount under frame near sewer connection or make your own.
A 5 inch x8 ft vinyl post cover, couple of top caps some fender washers bolts nuts and two truck tarp straps Use the tarp straps to hold the post top caps on
Put a 4 inch x 8 ft gutter inside it works great. Slide it out use the gutter to keep your hose up out of the dirt and gravel
YMMV
RoadrunnerII