Am I being silly?

[brman]

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?

[AndyUK]

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

Tack weld opposite points. Progress slowly and methodically, setting up as accurately as you can and measuring as you go. All you need is a stick welder, a grinder, and a tape measure.

Then level with epoxy. It's completely doable, loads of people on here have written about their experiences and how to do it. If you screw it up (like I did my first time) chip it off and try again.

Alternatively, skip the hard stuff and use aluminium profile. You'll be bolting your rails on in minutes rather than days or weeks (or you know, in my case, months).