There is a lite-weight cement material called "Structolite", which is a white powder with a kind of 'popped-rock' in it. It is half the weight of cement. But it's also half the strength and works best on small panels. 16X16 inches would be the largest practical use , I think, haven't pushed its limits. I mix it will portland cement , in parts 1:1, then add sand. Too much sand will make it crumbly. I made several 14X14 panels with it and EML ( metal lath ), made them very thin, barely enough plaster to 'key into' the lath. They don't weigh much more than a big dinner plate when dry. Two coats of lime morar will almost double the weight, but even so, they are very portable and hangable.