well it was raining today so thought more about this.
one thing that was holding me back was how to replace the 1 kilo of metal on the bottom of the glass as I didnt really know how it was fixed in place.
but it appears that it is just a clamp type setup.
with a small bit of 'over bent' 1.6mm alloy and some rubber squished between that and the glass then this little piece in the pic, is enough to hold up the 4kg of glass and original metal bit! very strong. definitely enough to stay attached when winding the window up and down.
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so now it seems i can replace the bottom steel with a couple of bit of alloy. one small triangular bit for the guide at the front of the windows and one snall bit for the middle of the window base to bolt on to the mazda winder.
that should be easily made with 200grams of alloy so a saving of 800grams!
combine that with 500grams from the winder and if i make the side guide bits from plastic or alloy i could save another 200-300grams.
so all up per door about 1.5kilo! awesome indeed! 6 kilo!