The easy way is to fit something similar to a roller blind.
A more substantial method is to use the same technique as a roll-top-desk.
Each inner face of the front side panels has a groove routed into it - obviously matching side for side and extending across the top and part way down the back.
Wooden slats are made to fit the grooves and glued onto a canvas backing.
It is best to make the grooves and make up the roller door before assembling the carcass.
You might get the idea from the drawing here...
http://chestofbooks.com/crafts/popular-mechanics/Mission-Furniture/images/Rolltop-Details.jpg
Peccavi, June 2009
