No, don't use aluminium, it can break very sudden and without warning (also most plastics).
Wear and tear is a saefty issue - even with steel. But with steel you mostly hear and feel it before something goes really wrong.
About the measurements: This are minimum sizes for proper safety. Everyone with Mechanical engineering knowledge should know what i mean.
Unfortunately, I cannot publish construction plans: The risk of someone accidentally killing themselves through misuse is too high.
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