The little tab for the belt buckle lock welds to the top and front at the cage as in this picture
The other part of the buckle with the push button welds to the top window net bar.
one of the sleeves should be welded to the top of the main hoop of the cage, you can just see it in the picture above the side cage padding, but it's actually an awkward idea.
Get yourself two 8mm rose joints (or hem joints in the US?), weld them to the end of the top and bottom net bars, or drill and tap them for extra adjustment, then weld a short stud or captive nut to the cage were the sleeve would be, bolt the rose jointed bar to the cage and when you unclip the buckle the window net simply swings down and you get out easily. I have change it to that now as it's easier to use then to pull the bars out of the sleeves when you want to get out. Especially if you're in a rush.
Frank