I've been wanting an excuse to share my experience with homemade ply
Varnished ply would be hard to spot in the water. Go to a ship
Chandlery and they will put you onto the right stuff of any colour.
I have built a LARGE TT from 4mm ply for training purposes. It's
3000mm by 50mm and weighs just under 5kg. This board ollows me to
put a beginner on the water in light wind,with a small kite,and keep
it safe. I've also found its great for "just having a go" myself. No
jumps, no spins, nothing fancy. Just having a go when everyone else
is sitting on the bank, twiddling their thumbs and toes. I use lots
of fins, especially on the heal side/middle of the board for getting
up wind. It seems to be a natural upwind board as I must consciously
turn the board downwind all the time.
Make sure you use good marine quality materials. Don't try to cut