I think wood could work but I would be concerned about longevity. Wood expands and contracts with temperature and humidity and that movement could push the window around enough to create a leak point in the exterior of the bus. When I did my windows I used 1" angle, half the weight of square tube, and welded a frame to the hat channels so the window trim rings had something rigid to attach to. It was really simple to do.
See pic below of my interior window support.
Good luck.