Get a proper cage, dont #### about with anything remotely iffy. When you put it on its roof and the welds fail crushing your skull you will wonder why you tried to save £££ on a £1200 cage.
Drink less beer, smoke less fags, ride your bike to work for a month, there are easier ways to save money than skimping on something which is SAFETY related.
Malcolm does a good job with his cages, been very happy with mine and admit its not really taken any knocks. But I know its there when I need it and if I put it on its side or on the roof I can still continue my journey 1000's of miles from home. All those mentioned (Whitbread, P&P, North Offroad) would have gotten my money, Malcolm was just closer to us and would spend time welding tabs for a modified ARB roofrack and roof tent. Im pretty sure North do this also. A friend got a cage from P&P and fitted it himself, its a quality piece of kit.
It will make your landy heavier on top (it does not take a genius to work that out) but you change/adapt your driving style to suit. The same as you would if you fitted a rack and threw a spare wheel ontop. Its just common sense and not really a bad thing.
G