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I need to fix this panel back on but not sure one the screws i need to use, the two in red boxes.
Phanteks does not publish specifications for the hardware used on their cases. I also don't see spare parts offered when I searched their website.
Chances are that it's an M2.5 or an M3 screw. You might get a metric screw assortment and try various sizes until you find a correct fit.
https://www.amazon.com/product/dp/B074KDWZNF