Wonderful project. You made 4? Are you making these commercially for sale?
You could make them lighter in a number of ways. Chromoly steel can be half as thick as standard steel, and can be close to the same weight as aluminum. One is also often tempted to over-build home-made projects.
You could also use thinner plywood (1/8" door skins?) for certain areas. Perhaps using a thinner front plate. I'm seeing some notes on plywood/foam composite panels.
For the thin plywood, build a solid, grooved frame that the thin plywood panels fit into.
Any place you can use fabric panels will save weight and materials.