Hey Alan, apologies for not getting much further on the model Ill get it sorted soon with the rest of the frame.
As for the wall thickness comment that would depend on a few things, are you thinking of EVER putting it in a competition? if the answer is yes to that then I would check the competition blue book rules for roll cage wall thickness as that would apply to the upper cage, if your not going to compete in it then its a down to personal safety and the trade off on weight - the wall thickness on my buggy is 2.5mm on a 32mm tube its very strong, bends very well without creasing on the former but has weight to it, a friends MEV has a much larger diameter tube in its frame but its something like 1mm to 1.5mm tube wall thickness and its very weak even he says he doesent want to be in a "big off" with it!!
To add some perspective I nose dived the buggy (in one of its off road trips) into a bloody massive hole that neither I nor my passenger actually saw, the suspension travelled its full length and the wheel arches passed the indicators on the front and we hit bump stop, we got a 50mm dint in the front 3mm thick alloy panel, the steering wheel bent back but nothing else and it drove away, when I stripped it down the front lower suspension arms competition rose joints (which are built like brick sh!t houses) were all bent but still functioning, the chassis which took the full brunt of the impact distributed the impact and nothing bent.
Now you know me I over engineer everything and would rather go heavier and stronger - but in my opinion dont go less than 2mm on the wall, Ive found the thinner larger diameter stuff does not bend as well on the formers and you tend to get ripples or flat spots on the curve.