I made a full length roof rack, used 2in square tube 42in long for the vertical beam and same tube for the 8ft long horizontal pieces, when I hopped on the horizontal piece it bent about a quarter inch,
I did a vertical piece between each window, I put two 3/8 inch bolts to hold each vertical piece to the frame between the windows, the pieces came down about 14in below the window gutter. I used a single 3/8in bolt to connect the horizontal piece to the right and left side vertical beams.
I bolted roofing sheet metal to the 44in long side beams to keep the sun off of the side of the bus.
The roof rack extends an extra foot from the front and back of the roof to protect the lights from branches.
The tube is probable 1/8th thick, again, get an 8ft piece and jump on it to test its strength.
my bus thread, https://www.skoolie.net/forums/showthread.php?t=8860&highlight=bubb