Hi, think the easier way is to solve your casement prob is to change a sliding window, less maintenance as compare to casement. But at one time you only have half of the opening space for sliding window. This way you have no problem installing the net. As for invisible grilles, is it for balcony area? and yet you wanted to do netting? hmm.. looks like you only have the options of "Aesthetics appearance" for invisible but unable to install netting. Or "practical usage" for aluminium grilles or sliding windows where you can easily attached the netting with magnets or double sided tape. Maybe you can consider aluminium grilles for a more cost saving option. i ever see a round tube grilles. it looks very slim compared to normal grilles. So for aesthetics wise, i think it will look just great.