Hey, U Turn How are ya!
Not ready to post pictures yet. One reason, the bus is a messy disaster and the other is (I need to learn how to post pictures.
Anyway, I will try to explain what I have done. Here goes!
Measure the opening after the window has been pulled, top to bottom side to side.
Then allow three quarters of an inch all the way arround the perimeter of your sheet metal.
Bend or brake this 3/4 towards the inside of the bus window opening.
You now have a (solid lip) to rivet on the inside. This will make the panel very strong. A little bit of sealer arround the inside, and a (nice bead on the outside) voila! a nice finish!
I painted the inside first and used urethane seam sealer on mine. Works for me, hope it helps you! Cheers