For enclosing the sides of the cockpit, consider Velcro, 'zip-lock' or zipper around the edge of the cockpit's side openings and flexible plastic sheeting sides, which are attached to the Velcro. There should probably be a few 'hard' attachment points so that a side sheet cannot pull of during flight. If the Velcro seal is complete than a slight air pressure inside the cockpit may hold these two sheets taught and slightly bowed outward. The attachment of these side enclosures will probably necessitate a addition to the vertical fin.