Yeah it's 5-6 years old and was part of a shed I demolished for a neighbour. The forge is made out of it and I still have a few left plus a ton of 6mm full sheets.
6mm is a bit thin but the support structure should take care of the weight and the ply will get a fair few tins of paint thrown over it. The supports are recycled fencing from another neighbours, err, fence. They are treated and have already proven their suitability for outside use having been a fence for ten years...