Caravan Air Conditioning

We nearly melted in Seville last year when sleeping in the caravan but we are not willing to fork out £2,500+ to fit a roof-mounted air con. unit on our Bailey Olympus.  I noticed on page 100 of the July CC magazine an advert for a portable unit (£450) which you hang out of the window when in use.  One of my concerns about this would be security, but does anyone have experience of these types of units or any other sub-£2,500 alternatives please.