I've used MDF for templates which it's great for but does deform and swell if it gets wet. Decent marine quality ply wouldn't have this problem so I had thought of using a combination of that and hardwood to build a strong structure but it could just become a bit cumbersome.

I can weld and it has to fit in a shed so steel makes more sense when I think about it.

I'm thinking 1" box section for the main frame.

Best get drawing.