i would definitely put some diagonal boards on that bottom box. those little corner pieces have a lever arm of a few inches to stop the beams from being torqued. if you put cross beams it has the entire width of the box as the lever arm. put them in both directions. 2 x 4s are dirt cheap. it may seem sturdy now, but after you climb up it a hundred times things start to loosen and weaken. if it were me, i'd put cross members on all three boxes, cause why not.
as others have said, that saw horse better be temporary cause it isn't going to do shit. i'd also be careful about how the very top of the ramp is connected to the top box. i can't see in the picture if you have bolts or what, but if it's just some drywall screws, you might want to add some robustness to it.
craftmanship is excellent though.
i've built all sorts of crap like this, and happen to be a mechanical engineer.