Is it doable?
Are they useable??
Are they more fun???
And does any particular shape work best???
Help me out 
Yeah its certainly do-able, my first plane was made from a skateboard deck.
Only problem I found was making it from a skateboard deck it had a rolled bottom and was difficult to put any useful concave in the bottom as a deck is fairly thin.
Saying that it does work and its a great way to get into bodysurfing.
what if you flipped it so that the curve was downward making it super concave??
Yeah I don’t see why that wouldn’t work. I used the part of the kick to provide some nose rocker on steeper waves.
Never used one but I’ve seen handplanes made from broken skate decks. No reason why it wouldn’t work well.
~Brian
korduroytv.com has a vid on it..
and there's a guy in marin, ca. doing it..
using an adj strap from leashes...