I'd like to build on my own a steel deck bed/sun lounger, where could I find instructions, measures, etc.?

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Measurements would be the easy task; just find a design you like in another material and take it's measurements.

Creating it will be the more labor intensive task.

I can't imagine wanting to BAKE on steel, but suggest Stainless if you Must go that way. It's also likely you'll end up with the most expensive Chaise on the planet.

You could go with galvanized conduit at an appropriate diameter, a pipe bender, webbing perhaps, some exterior grade coating, or just take the Chaise in when not using it.

Without knowing your desire for aesthetics, or any real Comfort, or thoughts about cushioning, you could fabricate it many ways, such as Tubing the framework, even using conduit collar connectors, steel mesh grating bolted on, etc.

I just surfed the WWW, which has always seemed a logical beginning,,, to me anyway, and find dozens in aluminum or powder coated steel, or galvanized, but all are factory produced.