First cut the crosspieces and rungs from wood; choice of wood is not especially important, even softwood will suffice even though birch is used here:
Once these components have been cut, grind their surfaces facing the little ladder bears to exactly same thickness. Also make sure that the width of the crosspiece is exactly correct, as the T-clamps are partially supported by it. The exact width of the the rungs is not at all important.
Next attach the beams, crosspiece and rungs together using clamps, upside down, and mark the placement of the holes using the holes in beams as a guide:
Disassemble the little ladder and drill small starter holes to the crosspiece and rungs:
At this point the rungs can be attached with screws:
Attach the little ladder to a vise and finish the square crosspiece hole with a drill and a chisel, until you manage to push a rectangular steel rivet through it:
Finish the little ladder by leveling off the crosspieces and rungs:
Fitting T-clamps Edit
There not much magic in attaching the T-clamps to the little ladder beams: they are simply hammered right next to the crosspiece, and as far down the little beams as possible:
The beauty of this alignment and T-clamp design visualized above becomes apparent when the clamps are attached to the case: even without any rivets, nails or metal plating the whole structure is extremely rigid.