I've already built the two side structures and counterweight box,
and the plywood probably wouldn't look as good.
I planned the base with double layered 2x4's so the components can be bolted together using T-nuts.
The counterweight is as heavy as whatever we put in the box, right now we have a spare bag of gravel in the driveway (20-25 lbs? Its labeled as .5 cubic feet) but we could get another bag to experiment once its all together. We are planning on throwing SCA rocks (4 tennis balls duct taped in a tetra) so the projectile won't be really heavy.