FWIW, I'm going with the milk crate (for now). I bought a plasticized laundry bag for a $1, then put the layers that I normally use in that, and then tried both crates with the bag-o-clothes. The milk crate just contains it all better, with room left over for additional or bulkier layers. And the zip ties I had that I used to mount blinkies to before were still there. Had to remove the seat wedge, but I just put in the bottom of the crate and then used its seat post attachment point to secure it to the inside wall of the crate. It bounces around a bit, but won't go flying if I take a spill.

Another reason I opted for the milk crate was I placed both crates on the rack and observed my brightest tail light (that is mounted to the rack) and the soda crate did block it somewhat.