You can make a board with quality 1/2" ply but you lose turning performance without any rocker. However the flat boards do work well for getting upwind. I would suggest making a plywood board with rocker if you have the time, or go all the way to fibreglass if you're really bored.
Plywood with rocker:
1) Buy 1/4" furniture or marine grade plywood, cut out 2 layers, leaving 2" buffer outside your design.
2) Stack the 2 layers and suspend between 2 objects. Add weights to flex ~3cm in the middle. Take off the weights, apply glue (waterproof expanding polyurethane glue or epoxy), add the weights again.
3) Remember the buffer around your design? Screw layers together in this buffer zone every 5cm, start in the centre and move to the tips, do both sides together. This helps bind the layers. Let glue dry
4) Cut out final design, sand and finish edges
5) Cut fin-holes, add inserts, seal wood with marine polyurethane or paint