Hey Palmer, I'd like to know if you have a parallelogram or a trapezoid, before recommending anything. If you have 2 parallel sides and the ends are out of whack, or vice-versa, that would make life easier. If not, you gotta start with the best face as a reference and mark or tape it. A straight bit say 2" just peeking out from the fence 1/32 or so and 1" high off the table, and the with best face against the miter gage make a light pass on the right side of the box. That should give you a second partial face at 90 degrees to the best face. Now it gets tricky, If you have a parallelogram rotate the box 180 degrees on the bottom and take another pass with the side opposite best against the miter gage. This reveal, or the lip or offset created, is the difference between "square" on the ends.
The same operation could just as easily be done on a table saw.
Now we have to remove any stock that is not in the same plane as the newly routed reveal.....you are on your own from here
.....belt sander? bandsaw?, hand plane? sub it out?
With the table router we are limited by the depth of cut.
If you have a trapezoid you'd have to keep best face against the miter gage at all times and flip the box upside down making one router pass from the top down and the other from the bottom up. The last side of the zoid needs to be made parallel to the best face and would require a positioning fixture to keep everything square. Since we can't pass the box between the bit and the fence
see post "Flying Wood", a fixture is required. BUT, there is still the issue of removing the stock that is not co-planer with the now squared reveal. Same issue as above. I have contributed absolutely nothing here,
I'm leaving now,