Get a piece of foam, carve it into the right shape. Add some spruce spars for extra structural suport. Cover it with fiber-glass or carbon-fiber, resin the whole thing and cover it with seran wrap to get it nice and smooth.
Then peil off the seran wrap an you'll have a nice smooth, strong, and earodynamic frame. And depending how well you squeeze out the excess resin - light too.
I've always wanted to build a robot that way. I <3 Fiber Glass
