what material do I use on the walls that will bend to the curve above the window

[Jeremy]

You can get lengths of ready-curved plywood in a variety of radii, and veneered in a variety of wood species, which is ideal for use in cabinetry and fit-out work in buses or boats (they are particularly used in boats, where it is good idea to avoid sharp corners and edges on the interior as much as possible).

If you open this PDF you will see some:

http://www.robbins.co.uk/Pdf%20Files/pl44_laminate_panels.pdf

The same company does bendy plywood too, plus all sorts of other useful products - I am getting all my Cherry veneered plywood from them.

As a possible cheaper option to the corner profiles mentioned above, I have also found that builders' merchants sell a range of pre-made curved 4mm ply lengths sold for 'boxing-in' pipework etc in houses. The radius on these may be too small for your particular application, but worth knowing about anyway:





Jeremy