roll bar tube

[shh120m]

sorry if this has already been asked (im sure it has) but i cant find it.

What diameter tube and what wall thickness do i need for the roll bar? Also does it have to be seamless? My old man has a mate whos an industrial pipe fitter and he has use of a mandrel bender at work, so i was going to have a crack at making one myself. Obviously i will probably have to compromise depending on what size formers he has but Im sort of toying with the idea of a full cage if the steel doesnt cost the earth.

thanks