I was going to run a bar from each side from one seat belt bolt to the other.
I've got a better idear though.
First I would lay out a peice of plexiglass or lexan and use a heat gun to bend it to mold to the roof's curve on the back.
Then remove the rear window and cut as you see it. I would run weatherstripping like that around the door openings on the edge and silicone it in place and some screws as well. Cut the plexi/lexan to fit cleanly on the weatherstripping without overlapping.
Then on the inside of the cab use the hold down found on jeep hoods every 8"s to pull the plexiglass tight to the cab and seal it.
I would weld in a support at the bend to prevent cave in of the plexi/lexan and to prevent flex.
The truck never sees speeds more then 45mph and thats only when I go down the long road to get to our offroad spots, it's mainly all residential roads of 35mph. So the chance of it flying off is low.
If i don't attempt this I will cut out a hole in the roof and do this same idea... and just remove the back window and use weatherstripping and wing nuts to remove it.