I designed this mount so it can work with both 15mm and 20mm extrusions so virtually any Kossel variant can use it. Rather than mounting on the Z tower, this one mounts on one of the lower base cross pieces. This keeps it out of the way - a nice feature if you transport your printer to events. My Smoothie mount (posted on Thingiverse) is mounted on the back tower of my Mini Kossel and I'm always bumping it into things!
Print with 3 perimeters and 2 to 3 shells. I printed mine with 33% infill in PLA. The holes are for 3mm screws. I printed some t-nuts that work with the 6mm wide slots in 2020 that take 3mm screws. Works great.
The ribs stiffen the frame. You will need to print or make/acquire 4 standoffs (mine are 8mm OD x 4mm tall). Attach the Duet with M3 x 10 self tapping screws with a washer under the head.
I may tweak this design a bit after I get my Kossel 250 commissioned but this seems to work well.