Above is a photo of the 14.5x14.5 inch roof vent cut out. My 1 inch foam is bonded top and bottom to 3mm birch (1/8in). This piece has no framing! I set it with about in inch purchase on each side and it held my 185 lbs easily. I could not feel any, but from the photo there is some slight deflection when I stood one footed. not sure of the weight (I will weigh when I get home) but it is easily lighter than 3/4 ply and probably lighter or the same as 1/2.
also, tanks required a lot more maintenance than dirt bikes. heavy does not = strong.
having said all that. if you want the flexability to be able to screw into your walls at any point in the future without having to worry about hitting framing. go with a single sheet of ply and be happy. lots of people have done that and it works well. I would strongly caution you against making a doubled up 3/4 wall though. it will make your tear VERY heavy and will not add any useful strength over a single 3/4.
