I think two ways of doing this... cut the kerfs on the outside of the panel or cut the kerfs on the inside....
on the out side will look nicer, on the inside may result in more insulation properties.
so, the cuts go lenghtwise in relation to the bus.. I would cut a strip about 6" wide and make cuts every inch in the area of the bend and press in place. Look at the effect the cuts have. Make more cuts or less, deeper cuts or shallower as you think. Play with a several strips as see what you think conforms the best.
plywood that is 3mm and five layers thick tough, strong, light weight, and not cheap, but really good flexy bendy. Look for "Baltic Birch" I have in the past seen sizes up to 4' x 16' if you dont want seams
You will have to google/search where can be purchased near you.
WATCH.. some may be quoted at 3 layer and 3mm but the better stuff is 3mm and five layer.