Ok, so you already have roof racks. What i could suggest is using Load stops with straps.
If you have square bars Thule 502 Load Stops (25cm high) or 503 (9cm high)
If you have wing or aero bars Thule 314 Load Stops (15cm high) or 315 (22 cm high)
You could use normal buckle straps or even ratchet straps.