Hey Neil following you around.
Should have asked me today 50mm Tube would be good I would say as they use 10mm on the frames for indoor spas may be over kill but thats the way I like doing things.
The indoor units are supported and a couple of the cheaper outside spas I looked at a couple of years ago had support on the bottom as well suing the ground as support shouldnt be a problem.
So I would build the frame with the 50mm and panel it as you said.