Update: If you don't care about pictures, here's a PDF with my step-by-step instructions.
I went for several years without a place in my band room for students to put their music. While I could have requested money to buy one in my budget, it didn't seem like it should be that expensive or time consuming to build. A new music folder cabinet runs $600-1000. Materials for mine were around $200.
The photos are actually of the first cabinet (90 slots) I did. The plans are for the second one (75 slots). The main difference is that the shelf on the bottom of the first cabinet was too short to be as useful as I had hoped. Making it taller allows the second cabinet to hold trumpet and trombone mutes on the bottom shelf.
The first cabinet, I glued and screwed in most sections. It is pretty sturdy. The second, I didn't glue as many parts, and it still turned out fine. I mainly used 1 1/4" wood screws. The casters are 3" and held in by 1 1/4" bolts with nuts and washers. It's a bit awkward to roll around, but is nice that it is movable. I finished the exposed edges with an iron on veneer.
After three years of use for the first cabinet, it is still holding up just fine. I've had to reglue the veneer in a couple of places, but overall I am quite happy with the cabinets
Addendum: I plan on modifying the second cabinet this summer to have 1.5" slots instead of 1.25". I'm getting some new folders for my oldest group and they are rather snug in the smaller slot. If I were doing everything over, I would still keep 1.25" slots on the first cabinet, because younger students don't need more space (except for the ones who save every book they have ever played and bring them to school for no reason, or the students who try to use their folder slot for their science book and math tests). Update: I finished my modifications and now have a cabinet with 21 1.5" slots instead of 25 1 1/4" slots.