Mine were side window louvers too. I didn't have any dimensions, just laid a sheet of plastic over a spare window, shaped it with a heat gun and then used that as the frame.
Cut it out and shaped each slat with the heat gun, plastic welded them on and sanded it down. Didn't have any money in the materials, since it was stuff I had laying around the shop anyway.
On mine, the slats continued to the end of the window (With a tab underneath to bolt on to the window/hinge)
I might try making another set again.