Ditto.....bike lock cable for the "security". Just make sure you have it wrapped or secured so that the lock nor the cable ends will end up against the paint. For actually attaching them to the roof rack I'd go with some of the cam-buckle style cheap straps from somewhere like Harbor Freight. A few wraps to take up the slack or even cut them to length and melt the ends. They're cheap but surprisingly strong.
For now mine ride inside between the seat back and my fridge. However....I would also like an outside storage solution on the cheap for when I actually use them and they're covered in mud. I keep a pair of those straps in my toolbox already. I'm not worried about security since it should only be for a short trip home (and I bought cheap knockoff traction boards).