I like that you added reflectors. I use a backpack and run bungees from the seat to the back of the rack, but I see your seat post isn't high enough from the rack to do what I do.

23 years ago I experimented with a milk crate and used two 2-inch thick 2-foot long velcro straps designed to hold fishing poles to a boat. They held the milk crate very secure and yet, were very easy to unstrap.
I don't use crates anymore, but I carry one of the straps in my backpack for when I need it.