I used the 3 part side mount slides - full extension - generic from Amazon, but had good reviews. I just checked, and the box is definitely a bit out of square, not bad, but that is most certainly contributing. I think it was square before I had to use a lot of shims to level the bottom and when I secured the box to the existing wall on the left, that likely pulled things out further. I did the slide measurements manually, and it was a pain trying to get them to stay perfectly while I installed. One other caveat with this piece is that I built it inside the closet, so I can't remove the box on it's own - it is bigger than the closet opening. I was trying to maximize drawer space and utilize the dead space on the right (which there are still a few more inches of.
So at this point if I'm going for "good enough" while also taking in how to do it differently next time, could I shave the drawer face and learn how to better square my box and install my slides for next time? The whole reason I picked this project is because I did my first drawer (singular) in a pantry shelf and messed it up so badly I felt I had to do something to incorporate everything I learned from that project.