2 layers glass on top
one layer 8 mm plywood
2 layers on bottom
maybe a bit soft.
but i would try first and add more later if you choose.
flat and floppy is fine for a light air board.
no need for rocker or concave unless you'd like to experiment.
perhaps soak it or sail it then clamp it against some bricks/blocks in the sun to coax some shape into it.
paint it once you want it to stop taking on water and protect edges and laminating resin
a 4 x 8 sheet of ply will net a few boards.
i dont really think this effort is worth the added expense of marine ply.
A/C plywood siding. or even luan door skins will work.
you get to choose.
if you choose to bond 2 layers of 8mm , you can mold in rocker and concave and get away with less glass.
0-1 layer of 120g glass on top and bottom.
straight sides, rounded tips, fins in the corners (or none)
155 x 45 sounds like a good place to start.
if you make it over sized, dont think twice about just cutting some off to suit your needs.
My advice would be not to use plywood. Glue some 300-400 kg/m3 wood strips 8x20x1500 instead.
Total of 280-350 g/m2 higher grade glass fabric (unidirectional and biaxial should dominate) on top and the same on bottom would be soft and fine. Marine plywood will add 1kg or more to your huge board. 3-4 cm continous rocker and 8 mm concave in the middle section for better plannig . Bend a core twice more lenghtwise before epoxy is cured.
I have made few boards of wood only in the past. 25mm thickness was very soft and comfortable. The latest twintips where 5 mm core with composite skins and they were not so soft... Most kiteboards waste weight on cores mainly. The second advice would be not to use more than 90 g/m2 top and 70 g/m2 bottom of fibers in the perpendicular direction on the board. I hardly manage to breake even thin cored boards with only 80 g/m2 top and 80g/m2 bottom perpendicular direction glass fibers. Some cracks developed on the top side in the middle section after heavy landings. My point was that you must use mainly unidirectional fibers. The core must resist only compresive forces between the skin layers basically. Unfortunately wood is most resistan to compression in lenghtwise direction... But in 8mm core it would be enough