I dont see why it would be that complex. All it would need is a tube of the right width, slotted along the length to grip the cover (or some other fixing arrangement), and two inserts for the ends of the tube to provide stub axles and a locator for the return spring which could either be a clock spring arrangement, or a cord and linear spring arrangement. Indeed if there were two blinds, each side of the slide the cord arrangement is prefered as the cord for each blind would be connected via a tensioning spring so one winds in as the other winds out.

Finding suitable cover material might be trickier.