In an effort to reduce costs on my Wilson i3 build (made from 20mm 8020 extruded aluminum), I started looking for alternatives for the rather expensive m5 T-slot nuts used in 20x20mm extrusion. I attempted standard square m5 nuts (too tall), standard hex nuts (again too tall), and finally m5 jam nuts (about $3 for 100 at fastenal). Jam nuts work, but they spin in the extrusion slot, and also don't sit flush in the slot, leading to potential issues with cross threading. This sled performs 2 functions, preventing the nut from spinning in the extrusion, and holding it flush, allowing for easy initial thread capture. These are a tight fit, by design - you may need to lower your flowrate (not your feedrate, your flowrate), even then I usually have to tap the jam nuts in with a light tap with a hammer. Enjoy!