Yeah Wills boat Jive was previously called Smik was built of 2mm ply and comes in under weight at around 50kg.His main deck was 3mm I believe.

My new 3mm deck is reinforced with 120 gram glass on the inside. The downside is it does tend to flex and I have small hairline cracks in my paint job. Possibly by using 4mm with a layer of glass or 3mm with 2 layers of glass it would be alot stiffer but stll a few kgs lighter.

I'd also like to point out removing the old deck is not easy and I had a few areas damaged due to this. All repairable and not structural but If you get unlucky it could create a can of worms.