pic: my version of a wooden chassis

[MrForbes]

There is no need at all for the steel corner brackets if you use the triangle gusset plates on the top. All they do is add weight.

The 2x6 sides are way overkill, if you think you need some more strength at the axle attachment points, you could double the plywood at that area, but it really isn't needed. The cantilevered axles are short, and if they are made of 1/2" bolts then they are very strong, and the nuts and washers used to attach them spread the load out over a large area of wood. Also birch plywood is harder and stronger (because it's laminated from plies that go different directions) than a fir 2x6.

There is a lot of engineering thought behind the design we came up with....