Were I to make the repair I would:
a. Buy some 16 ga aluminum diamond plate.
b. Cut two pieces the width of the tube (make it longer than the opening as you need to make up for the curveature of the ends...).
c. Cut a four pieces of 2"X4" three inches long.
d. Center the cut aluminum over the opening in the nerf bar.
e. Using a large vice grip pliers, compress the aluminum down to the tube and lock the pliers.
f. Using the wooden block made in step "C" and a hammer, deform the aluminum to fit the curve of the tube opening.
g. Use a "C" clamp to hold the ends down into the nerf bar.
h. Using a permenant marker, trace the outline on the underside excess aluminum plate.
i. Remove the clamps and the plate.
j. If done right, the plate should conform to the opening in the nerf bar.
k. Measure and drill holes in the plate to affix the plate to the nerf bar.
l. Cut the aluminum plate to match the outline drawn in step "h".
Now if you want to do a better job of this...I'd weld a tab on the top of the nerf bar openings, and another mid way accross the opening and drill the tabs for screws. Then drill matching holes in the aluminum plate.
This addition will help to draw the ends of the plate to the tube and secure it in place.
Before screwing the plate in place apply a 1/4" bead or Silicon (RTV, sealant, etc.) to the nerf bar. As the plate is drawn to the nerf bar with the screws the sealant will spread and seal the seam. After the sealant cures trim the excess and you're done.
See... easy as pie!