Thanks, that is very useful and definitely gives me food for thought.

It does give me a problem though. From my experience with aluminium I don't think a basic extruded design is going to ideal either. Which is probably why you recommend steel?

My problem is that I am concerned how I would make a decent steel frame with the tools I have available and without excessive work. Specifically, steel sections are not generally dimensionally accurate so getting rails coplanar without milling or resin leveling. I am also concerned that if I weld then things will distort (I am very much an amateur welder!).
What I don't want to do it invest time in a steel frame which ultimately ends up looking like a pringle ;(
I realise this might sound contradictory when I was earlier talking about plywood but at least with plywood I can easily adjust with files, planes and sanding.

Maybe that should have been the question? How best to make a steel frame that is accurate enough?