Well I built one a while back and this is how it turned out.
For the main structure itself, realize your back is not flat as a board. So build it with a curvature so that it will conform to the shape of your back.
Next, it depends on if you want it to function as one solid life bar or you want a way to indicate the reviving or loss of life bars. Depending on which I can then give an explanation for it.
Now how you will wear it, you can strap it on with a elastic band and wear it like a backpack.
Any questions or concerns?
I hope this helped.