Strap them on with nylon cable ties - it won't stop the thieves if they're really determined but it usually acts as a deterrent. The ties are available in a choice of colours, clear, blue, black, white and of course silver. You can even get pink or yellow ones if it takes your fancy. Costs vary - I've seen the silver ones at £2 a time (daylight robbery if you ask me) or you can buy multipacks from £2 per hundred.- if you're in the electrical trade. I never travel in the car without half a dozen of them cos they're really handy for temporary fixes like tying up broken exhausts, etc and they can be joined together to make really long ones.