Back when was skateboarding a long tme ago I know boards where 7 ply laminated decks. They use a hydraulic press to laminate them so obviously you cannot do that but you could try to mimick this by using a car. I would not use a solid piece of wood because that wil most likely break. Plywood is durable and preferable because the layers of wood are put together in an alternating pattern so you do not have all the grain of the deck in the same direction which will give it strength. If I were you I would use 1/2" plywood or maybe even 3/4" (but that seems like overkill) and get some 6 oz fiberglass cloth and epoxy resin. If you put two layers of 6 oz cloth on both sides of the board it will be moisture tight and extremely strong. It will flex in the middle but I do not think it will break. You could always reinforce it lengthwise with a flat 1" wide piece ofsteel readily available at Home depot.
I think your main problem will be with the deck flexing too much and rubbing the ground in the middle. 7 feet is a lot of room for flex between the trucks. I think you should stay away from plywood and look into some other sturdier options.