I'm not trying to push ripping sheets of plywood, just passing along a technique some use. The same with the leg methodology. I've only used inexpensive 1x3s/2x4s from HD or Lowe's and found both to be very uneven. Even the poplar 1x4s I've looked at were very uneven.
Anyway, here's a couple of more renditions of both the layout and modules.
The 1st photo shows the layout a possible configuration of 18"x48" and 24"x48" modules. As you can imagine, this would then be covered with plywood decking that would be cut to shape. Module options are in the upper right. Obviously, some custom sized modules would be needed in places and could be used in other places to fit everything evenly.
The 2nd photo shows a 3D view of the layout with track height set at 36", note the modules peeking through.
The 3rd photo shows versions of the modules with extensions:
The lower module is 18"x48". It includes the full set of legs complete with cleats, etc. One set of legs for the extension doesn't have the cleats because the leg would be sandwiched with the full leg on the main module.
The middle module is 24"x48". It includes the cleats on one end, but no leg on the other because the end frame rails would be attached to each other.
The upper module is also 24"x48", but could easily be trimmed to 18"x48". It has no cleats because the legs are full height and attached inside the framing. Truth be told, that's probably all you really need, especially if you add the optional shelving.
I've included the layout file as well as the module file so you can take a closer look at just the modules if you want.