Quote JaeXD:
There are sizes/cuts of balsa wood that can be cut with craft blades. Up to 10mm thick can be cut with a blade, but it'll have to be quite sharp.

You can then layer up these like one of those homer simpson heads that's made up in layers; effectiveley printing in 3D.

Its really easy to sand down too, as all you need is some fine grade sandpaper and a sanding block (can be anything solid and flat, like a nokia 3210)

Once its built in balsa, use a PVA/Water glue mix to give it some outer strength and to really meld it all together. Using a sheet of acrylic (3-5mm) for the central layer would give it strength and flexibility boost.

That's one method.

Paper maché is another. However you will get a more textured finish without a smooth surface that you are able to sand.

And if you truly want lightness, get a sheet of flexible plastic, roughly 1mm in thickness. Cut out a "net" for the key and build it like a giant box or some kind of packaging. May have to be done in parts with much practise, but once its done it'll weigh virtually nothing. Its the same principle with making a net for any 3D shape; cube, pyramid, etc.