I used to build kites with about 2' wingspan. I used cheap silk cloth for them. This is very lightweight and pretty robust. It can be colored in bright colors without adding significant weight. The effectiveness as signaling device will depend on the circumstances. If kites are unexpected in the area they are going to attract attention. When someone is already looking for you it might help to put him on the right track.
I donīt think that writing SOS on it will help too much. The kite is visible long before the writing can be read. If it doesnīt raise interest at this stage itīs not likely that the person is coming closer to investigate. OTOH it wont hurt , so it could be worth trying.
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If it isnīt broken, it doesnīt have enough features yet.