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More contemplation due
You already have time and money sunk in the project. Do it right, do it once.
Marine ply is cheap (<£50 for 12mm thick) and should last a couple of years with no maintenance. You'd need to do the maths on what thickness you need, 6mm is going to be roughly the same weight as 2mm aluminium.
I think the answer is probably Aluminium. Your 2040×1000 sheet in 2mm 5-bar treadplate would be just over £150 delivered and 11kg
The advantage over plastic is high elastic modulus and high strength, so you can use a much thinner sheet.