what would be the best/cheapest/easiest/most effective method?
I don't know if my method is the best, but is sure is the cheapest and very easy. I am not keeping the original ceiling and I had all these stainless steel panels left over from ripping the ceiling out, so I just cut them down to fit, calked between the bus and panel real good and used plenty of fasteners, I also added some additional bracing to help stiffen the panels. I don't know if I would have enough to cover all the windows, but I have enough to do the ones I am eliminating. I will let you know in a couple of years how they are holding up