If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
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I think that optimal design would include the red heart that is placed on driver licences (in most American states) and on NHS cards (in the UK), plus the words "Organ donor". You might also want to include your organ donor ID, but you might not... in the US this is (sometimes? usually?) your driver's licence number, which may not be something you need strangers to see when you are at the beach.
My understanding is that if you do not specify otherwise, it is assumed that they can take any organ they need, but if you wanted to clarify (or were worried that your relatives my get greedy about the parts you get buried with), I would expect that the words "no limitations" would be sufficient to allow the hospital to take any skin, eyes, etc., they feel they have a use for.
Optimal wording may be less important than optimal placement. I would assume on the chest over the heart would be least likely to be destroyed in an accident / most likely to be seen by first responders... Plus, if that is destroyed, the best organs are also likely to be damaged. However, if you want optimal, you should really get a set of tattoos -- one for the chest, one for the stomach, and one for the neck(?).
Damn, I'd better just get a card then. Thank you!