I *think* you can buy something that might be called an A frame - try ringing Forest camping - actually I've just looked on their website and they have something called a verandah bar here not sure if it's the same thing?
Otherwise, I'd probably try to construct something with clingons and some king poles - the clingons would attach to the door and they you'd have to find a way of connecting these to the king poles - shouldn't be too difficult...
Amazing family weekend with old steam engines, classic car displays, market stalls, and full catering and bar. And camping on site - Save £25 by booking in advance.