I cant help with the louvers, but back when i had my Scarab i wanted to run with the hatch cracked open a bit to let more air in....never did cause i heard too many horror stories about the wind ripping them off!
So i thought why not make some straps that would through bolt from the hatch to the firewall inside the engine compartment? You can crack the hatch open and have the tention from the hatch lift and the straps keep the hatch inplace and keep it from "flying" away! Easy fix for LOTS of extra air.....you get to listen to the engine too!
Only drawback i see is if you had people sitting in the back seat with long hair! But then again my thoughts were if i'm running with the hatch cracked open then no one would be sitting down anyway!